Online Etymology Dictionary

theatrical (adj.): 1550s, "pertaining to the theater;" see theater + -ical. Sense of "stagy, histrionic" is attested from 1709. Related: Theatrically; theatricality.
theatrics (n.): 1807, "matters pertaining to the stage," from noun use of theatric (adj.) "pertaining to the theater" (1706), from theater. Meaning "theatrical behavior" is attested from 1929, American English.
Thebaid (n.): 1727, "district around Thebes (in Egypt)," formerly haunted by hermits and ascetics. Also, "pertaining to (Boeotian) Thebes" in Greece, especially in reference to the poem by Statius.
theco-: before vowels thec-, word-forming element used in botany and zoology with the sense "case, capsule," from Latinized combining form of Greek theke "case, receptacle," from root of tithenai "to put, place" (see theme).
thee (pron.): Old English þe (accusative and dative singular of þu "thou"), from Proto-Germanic *theke (source also of Old Frisian thi, Middle Dutch di, Old High German dih, German dich, Old Norse þik, Norwegian deg, Gothic þuk), from PIE *tege-, accusative of root *tu-, second person singular pronoun (see thou). The verb meaning "to use the pronoun 'thee' to someone" is recorded from 1662, in connection with the rise of Quakerism.

In Middle English, people began to use plural forms in all cases, at first as a sign of respect to superiors, then as a courtesy to equals. By the 1600s, the singular forms had come to represent familiarity and lack of status, and fell from use except in the case of a few dialects, notably in the north of England. People in Lancashire north of the Rossendale Forest and Yorkshire formerly were noted for use of the singular second person pronouns tha (nom.) and thee (acc.). For religious reasons (Christian equality of persons, but also justified as grammatically correct), the Quakers also retained the familiar forms.
Thou and Thee was a sore cut to proud flesh and them that sought self-honour, who, though they would say it to God and Christ, could not endure to have it said to themselves. So that we were often beaten and abused, and sometimes in danger of our lives, for using those words to some proud men, who would say, "What! you ill-bred clown, do you Thou me?" as though Christian breeding consisted in saying You to one; which is contrary to all their grammar and teaching books, by which they instructed their youth. [George Fox's journal, 1661]

While the Quakers originally adopted "thee" and "thou" on account of their grammatical correctness, they soon fell into the careless habit of using "thee," the objective, instead of "thou," the nominative. Common illustrations are: "How does thee do?" or "Will thee," etc. [George Fox Tucker, "A Quaker Home," Boston, 1891]
theft (n.): mid-13c., from Old English þeofð (West Saxon þiefð) "theft," from Proto-Germanic *theubitho (source also of Old Frisian thiufthe, Old Norse þyfð), from *theubaz "thief" (see thief) + abstract formative suffix *-itha (cognate with Latin -itatem; see -th (2)).
thegn (n.): "military tenant of an Anglo-Saxon king," 1848, a modern revival of an Old English form; see thane.
their (pron.): plural possessive pronoun, c. 1200, from Old Norse þierra "of them," genitive of plural personal and demonstrative pronoun þeir "they" (see they). Replaced Old English hiera. As an adjective from late 14c. Use with singular objects, scorned by grammarians, is attested from c. 1300, and OED quotes this in Fielding, Goldsmith, Sydney Smith, and Thackeray. Theirs (c. 1300) is a double possessive. Alternative form theirn (1836) is attested in Midlands and southern dialect in U.K. and the Ozarks region of the U.S.
theirs (pron.): possessive pronoun, "their own," early 14c., from their + possessive -s, on analogy of his, etc. In form, a double possessive.
theirself (pron.): emphatic plural pronoun, c. 1300, from their + self, with self, originally an inflected adjective, treated as a noun with a meaning "person." Related: Theirselves.
theism (n.): 1670s, "belief in a deity or deities," (as opposed to atheism); by 1711 as "belief in one god" (as opposed to polytheism); by 1714 as "belief in the existence of God as creator and ruler of the universe" (as opposed to deism), the usual modern sense; see theist + -ism.
Theism assumes a living relation of God to his creatures, but does not define it. It differs from deism in that the latter is negative and involves a denial of revelation, while the former is affirmative, and underlies Christianity. One may be a theist and not be a Christian, but he cannot be a Christian and not be a theist. [Century Dictionary]
theist (n.): 1660s, from Greek theos "god" (see theo-) + -ist. The original senses was that later reserved to deist: "one who believes in a transcendent god but denies revelation." Later in 18c. theist was contrasted with deist, as believing in a personal God and allowing the possibility of revelation.
theistic (adj.): 1780, from theist + -ic. Related: Theistical (1690s); theistically.
them (pron.): third person plural pronoun, c. 1200, from Old Norse þeim, dative of plural personal and demonstrative pronoun þeir (see they). Replaced Old English cognate him, heom.
thematic (adj.): 1690s, in logic, from Greek thematikos, from thema (genitive thematos; see theme). From 1871 of writing or discourse. Related: Thematical; thematically.
theme (n.): early 14c., "subject or topic on which a person writes or speaks," from Old French tesme (13c., with silent -s- "indicating vowel length" [OED], Modern French thème) and directly from Latin thema "a subject, thesis," from Greek thema "a proposition, subject, deposit," literally "something set down," from root of tithenai "put down, place," from PIE *dhe-mn, from root *dhe- "to put, to do" (see factitious). Meaning "school essay" is from 1540s. Extension to music first recorded 1670s; theme song first attested 1929. Theme park is from 1960.
Themis: Greek goddess of law and justice, the name means "custom, right," literally "that which is laid down or established" (by custom); also "laws, ordinances," but closer in sense to Latin ius (see jurist) than to lex (see legal) ; related to thema "proposition; that which is placed" (see theme).
Themistocles: name of great Athenian political leader, from Greek Themistokles, literally "famed in law and right," from themis "custom, law, right" (see Themis) + -kles (see Damocles).
themselves (pron.): mid-15c. in northern dialect, standard from 1540s, alteration of Middle English tham-self, emphatic plural pronoun, also reciprocal pronoun (14c.); see them + self, with self, originally an inflected adjective, treated as a noun with a meaning "person" and pluralized. Displacing Old English heom selfum (dative). Themself returned late 20c. as some writers took to replacing himself with gender-neutral everyone, anyone, etc.
then (adv.): adverb of time, Old English þanne, þænne, þonne, from Proto-Germanic *thana- (source also of Old Frisian thenne, Old Saxon thanna, Dutch dan, Old High German danne, German dann), from PIE demonstrative pronoun root *to- (see the).

As a conjunction, "in that case, therefore," in Old English. As an adjective, "being at that time," from 1580s. As a noun from early 14c. For further sense development, see than. Similar evolutions in other Germanic languages; Dutch uses dan in both senses, but German has dann (adv.) "then," denn (conj.) "than." Now and then "at various times" is attested from 1550s; earlier then and then (c. 1200).
thence (adv.): late 13c., from Old English þanone, þanon "from that place" + adverbial genitive -es. Old English þanone/þanon is from Proto-Germanic *thanana (source also of Old Saxon thanana, Old Norse þana, Old Frisian thana, Old High German danana, German von dannen), related obscurely to the root of then, and ultimately from PIE demonstrative base *to- (see the). Written with -c- to indicate a voiceless "s" sound. Meaning "from that time" is from late 14c.; sense of "for that reason" is from 1650s. From thence is redundant.
thenceforth (adv.): late 14c., from thence + forth.
thenceforward (adv.): mid-15c., from thence + forward.
theo-: word-forming element meaning "god, gods, God," from comb. form of Greek theos "god," from PIE root *dhes-, root of words applied to various religious concepts, such as Latin feriae "holidays," festus "festive," fanum "temple."
Theobald: masc. proper name, from Medieval Latin Theobaldus, from Old High German Theudobald, from theuda "folk, people" (see Teutonic) + bald "bold" (see bold). Form influenced in Medieval Latin by the many Greek-derived names beginning in Theo-.
theocentric (adj.): 1856, from theo- + -centric.
theocracy (n.): 1737; earlier as un-Latinized theocraty (1620s), "form of government in which God is recognized as supreme ruler and his laws form the statute book," originally of the sacerdotal government of Israel before the rise of kings, from later Greek theokratia (Josephus), literally "the rule of God," from theos "god" (see theo-) + kratos "a rule, regime, strength" (see -cracy). Meaning "priestly or religious body wielding political and civil power" is recorded from 1825. Related: Theocratic (1741).
theocrat (n.): 1827, "a ruler in the name of God," from Greek theos "god" (see theo-) + -crat, from aristocrat, etc. From 1843 as "one who favors a system of theocracy." Theocratist was the name of a publication begun in 1828 "to maintain the essential relation which subsists between religion and politics," and might be used in the sense "one who emphasizes divine authority over reason and individual freedom and who explains social order as a revelation from God."
theodicy (n.): "vindication of divine justice," 1771, from French théodicée, title of a 1710 work by Leibniz to prove the justice of God in a world with much moral and physical evil, from Greek theos "god" (see theo-) + dike "custom, usage; justice, right; court case," from PIE *dika-, from root *deik- (see diction). Related: Theodicean.
theodolite (n.): surveying instrument, 1570s, of unknown origin (see OED for discussion). "The word has a Gr[eek] semblance, but no obvious Gr[eek] basis" [Century Dictionary].
Theodore: masc. proper name, from Latin Theodorus, from Greek Theodoros, literally "gift of god," from theos "god" (see theo-) + doron "gift" (see date (n.1)). The fem. form is Theodora.
Theodoric: masc. proper name, from Late Latin Theodoricus, from Gothic, literally "ruler of the people," from Gothic þiuda "people" (see Teutonic) + *reiks "ruler" (see Reich). For spelling, see Theobald. The French form of the name, via the Franks, is Thierry.
Theodosia: fem. proper name, from Greek Theodosia, literally "gift of the gods," from theos "god" (see theo-) + dosis "a giving," from stem of didonai "to give" (see date (n.1)).
theogony (n.): 1610s, "the account of the birth or genealogy of the gods," from Greek theogonia "generation or genealogy of the gods," from theos "a god" (see theo-) + -gonia "a begetting," from gonos "birth" (see genus).
theologian (n.): late 15c., from Old French theologien (14c.), from theologie; see theology. A petty or paltry theologist is a theologaster (1620s), used in Medieval Latin by Martin Luther (1518).
theological (adj.): early 15c., "pertaining to theology," from Medieval Latin theologicalis, from Latin theologicus, from theologia (see theology). Related: Theologically.
theologist (n.): 1630s, from Medieval Latin theologista, agent noun from theologizare, from Latin theologia (see theology). Earlier in the same sense was theologician (1550s).
theology (n.): mid-14c., "the science of religion, study of God and his relationship to humanity," from Old French theologie "philosophical study of Christian doctrine; Scripture" (14c.), from Latin theologia, from Greek theologia "an account of the gods," from theologos "one discoursing on the gods," from theos "god" (see theo-) + -logos "treating of" (see -logy). Meaning "a particular system of theology" is from 1660s.
Theology moves back and forth between two poles, the eternal truth of its foundations and the temporal situation in which the eternal truth must be received. [Paul Tillich, "Systematic Theology," 1951]
theophany (n.): "an appearance of God to man," 1630s, from Late Latin theophania, from Greek theos "god" (see theo-) + phainein "to show" (see phantasm). In Middle English "Epiphany" (late 12c.). Ancient Greek Theophaneia was the name of a festival at Delphi during which the statues of Apollo and other gods were displayed to the public.
Theophilus: masc. proper name, Latinized form of Greek Theophilos, literally "dear to God; loved by the gods," from theos "god" (see theo-) + philos "loved, beloved" (see -phile).
theorem (n.): 1550s, from Middle French théorème (16c.) and directly from Late Latin theorema, from Greek theorema "spectacle, sight," in Euclid "proposition to be proved," literally "that which is looked at," from theorein "to look at, behold" (see theory).
theoretical (adj.): 1610s, "contemplative," with -al (1) + Late Latin theoreticus "of or pertaining to theory," from Greek theoretikos "contemplative, speculative, pertaining to theory" (by Aristotle contrasted to praktikos), from theoretos "that may be seen or considered," from theorein "to consider, look at" (see theory). Meaning "pertaining to theory, making deductions from theory not from fact" (opposed to practical) is from 1650s; earlier in this sense was theorical (c. 1500). Meaning "ideal, hypothetical" is from 1790s (implied in theoretically). Related: theoretician.
theorist (n.): "one given to theory and speculation," 1590s; see theory + -ist.
theorize (v.): 1630s, perhaps a formation in English from theory + -ize. Related: Theorized; theorizing.
theory (n.): 1590s, "conception, mental scheme," from Late Latin theoria (Jerome), from Greek theoria "contemplation, speculation; a looking at, viewing; a sight, show, spectacle, things looked at," from theorein "to consider, speculate, look at," from theoros "spectator," from thea "a view" (see theater) + horan "to see," possibly from PIE root *wer- (4) "to perceive" (see ward (n.)).

Earlier in this sense was theorical (n.), late 15c. Sense of "principles or methods of a science or art" (rather than its practice) is first recorded 1610s (as in music theory, which is the science of musical composition, apart from practice or performance). Sense of "an intelligible explanation based on observation and reasoning" is from 1630s.
theosophy (n.): 1640s (implied in theosophical), "knowledge of divine things obtained through mystic study," from Medieval Latin theosophia (c.880), from Late Greek theosophia (c.500) "wisdom concerning God or things divine," from Greek theosophos "one wise about God," from theos "god" (see theo-) + sophia "skill, knowledge of, acquaintance with; philosophy," from sophos "wise, learned" (see sophist).

Applied variously over the years, including to the followers of Swedenborg. Taken as the name of a modern philosophical system (sometimes called Esoteric Buddhism), founded in New York 1875 as "Theosophical Society" by Madame Blavatsky and others, which has elements of Hinduism and Buddhism and claims supernatural knowledge of the divinity and his words deeper than that obtained from empiricism. Related: Theosophist.
ther-: often thero-, word-forming element meaning "beast," from comb. form of Greek ther "wild beast, beast of prey," from PIE *ghwer- "wild beast" (source also of Latin ferus "wild," ferox "fierce;" see fierce). Also therio-, from Greek therion.
therapeutic (adj.): pertaining to the healing of disease, 1640s, from Modern Latin therapeuticus "curing, healing," from Greek therapeutikos, from therapeutein "to cure, treat medically," primarily "do service, take care of, provide for," of unknown origin, related to therapon "attendant." Therapeutic was used from 1540s as a noun meaning "the branch of medicine concerned with treatment of disease." Related: Therapeutical (c. 1600).
therapist (n.): 1880, from therapy + -ist; earlier was therapeutist (1816). Especially of psychotherapy practitioners from c. 1930s.
therapy (n.): 1846, "medical treatment of disease," from Modern Latin therapia, from Greek therapeia "curing, healing, service done to the sick; a waiting on, service," from therapeuein "to cure, treat medically," literally "attend, do service, take care of" (see therapeutic).