Online Etymology Dictionary

pro rata: 1570s, from Latin pro rata (parte) "according to the calculated (share)," from pro "for" (see pro-) + rata, ablative singular of ratus, past participle of reri "to count, reckon" (see rate (n.)).
pro tanto: Latin, literally "for so much; to such an extent;" see pro- + tantamount.
pro tem: 1828, short for pro tempore.
pro tempore: Latin, literally "for the time (being)." Abbreviated form pro tem is attested by 1828.
pro-: word-forming element meaning "forward, forth, toward the front" (as in proclaim, proceed); "beforehand, in advance" (prohibit, provide); "taking care of" (procure); "in place of, on behalf of" (proconsul, pronoun); from Latin pro "on behalf of, in place of, before, for, in exchange for, just as," which also was used as a prefix.

Also in some cases from cognate Greek pro "before, in front of, sooner," which also was used in Greek as a prefix (as in problem). Both the Latin and Greek words are from PIE *pro- (source also of Sanskrit pra- "before, forward, forth;" Gothic faura "before," Old English fore "before, for, on account of," fram "forward, from;" Old Irish roar "enough"), extended form of root *per- (1) "forward, through" (see per).

The common modern sense "in favor of, favoring" (pro-independence, pro-fluoridation, pro-Soviet, etc.) was not in classical Latin and is attested in English from early 19c.
pro-am (adj.): "including professional and amateur players," 1949.
pro-choice (adj.): "favoring a right to abortion," 1975, from pro- + choice.
pro-life (adj.): "opposed to abortion," first attested 1976, from pro- + life. Hostile alternative anti-choice attested 1978 in Ms. magazine (see pro-choice).
What hypocrisy to call such anti-humanitarian people 'pro-life.' Call them what they are -- antichoice. ["Ms.," Oct. 8, 1978]
pro-slavery (adj.): 1825, from pro- + slavery.
proactive (adj.): also pro-active, of persons or policies, as an opposition to reactive, 1921, from pro- + active. From 1933, in psychology (learning theory). Related: Proactively; proactiveness; proactivity.
probabilistic (adj.): 1855, in a theological sense, from probabilist (1650s, from French probabiliste, 17c., from Latin probabilis, see probable) + -ic. Meaning "pertaining to probability" is from 1951. Related: Probabilism.
probability (n.): mid-15c., "quality of being probable," from Old French probabilite (14c.) and directly from Latin probabilitatem (nominative probabilitas) "credibility, probability," from probabilis (see probable). Meaning "something likely to be true" is from 1570s; mathematical sense is from 1718.
probable (adj.): late 14c., from Old French probable "provable, demonstrable" (14c.), from Latin probabilis "worthy of approval, pleasing, agreeable, acceptable; provable, that may be assumed to be believed, credible," from probare "to try, to test" (see prove). Probable cause as a legal term is attested from 1670s.
probably (adv.): mid-15c., "plausibly," from probable + -ly (2). As a general purpose qualifier, 1610s.
proband (n.): 1929, from Latin probandus, gerundive of probare "to make good, esteem good; make credible, show, prove, demonstrate" (see prove).
probate (n.): "official proving of a will," c. 1400, from Latin probatum "a thing proved," neuter of probatus "tried, tested, proved," past participle of probare "to try, test, prove" (see prove).
probate (v.): 1560s, "to prove," from probate (n.) or from Latin probatus, past participle of probare. Specific sense of "prove the genuineness of a will" is from 1792. Related: Probated; probating.
probation (n.): early 15c., "trial, experiment, test," from Old French probacion "proof, evidence" (14c., Modern French probation) and directly from Latin probationem (nominative probatio) "approval, assent; a proving, trial, inspection, examination," noun of action from past participle stem of probare "to test" (see prove). Meaning "testing of a person's conduct" (especially as a trial period for membership) is from early 15c.; theological sense first recorded 1520s; criminal justice sense is recorded by 1866. As a verb from 1640s. Related: Probationer; probationary.
probative (adj.): mid-15c., from Latin probativus "belonging to proof," from probat-, past participle stem of probare (see prove).
probe (n.): early 15c., "instrument for exploring wounds, etc.," also "an examination," from Medieval Latin proba "examination," in Late Latin "a test, proof," from Latin probare (see prove). Meaning "act of probing" is 1890, from the verb; figurative sense of "penetrating investigation" is from 1903. Meaning "small, unmanned exploratory craft" is attested from 1953.
probe (v.): 1640s, originally figurative; "to search thoroughly, interrogate;" from probe (n.) and partly from Latin probare. Literal sense of "to examine with a probe" is from 1680s. Related: Probed; probing; probingly.
probity (n.): early 15c., from Middle French probité, from Latin probitatem (nominative probitas) "uprightness, honesty," from probus "worthy, good" (see prove).
problem (n.): late 14c., "a difficult question proposed for solution," from Old French problème (14c.) and directly from Latin problema, from Greek problema "a task, that which is proposed, a question;" also "anything projecting, headland, promontory; fence, barrier;" also "a problem in geometry," literally "thing put forward," from proballein "propose," from pro "forward" (see pro-) + ballein "to throw" (see ballistics).

Meaning "a difficulty" is mid-15c. Mathematical sense is from 1560s in English. Problem child first recorded 1920. Phrase _______ problem in reference to a persistent and seemingly insoluble difficulty is attested at least from 1882, in Jewish problem. Response no problem "that is acceptable; that can be done without difficulty" is recorded from 1968.
problematic (adj.): c. 1600, "doubtful, questionable," from French problematique (15c.), from Late Latin problematicus, from Greek problematikos "pertaining to a problem," from problematos, genitive of problema (see problem). Specific sense in logic, differentiating what is possible from what is necessarily true, is from 1610s. Related: Problematical (1560s); problematically.
proboscis (n.): c. 1600, "elephant's trunk," from Latin proboscis (Pliny), from Greek proboskis "elephant's trunk," literally "means for taking food," from pro "forward" (see pro-) + boskein "to nourish, feed," from boskesthai "graze, be fed," from stem *bot- (source of botane "grass, fodder;" see botanic).
procaine (n.): 1918, from pro- + cocaine.
procedural (adj.): 1876, from procedure + -al (1). Related: Procedurally.
procedure (n.): 1610s, "fact or manner of proceeding," from French procédure "manner of proceeding" (c. 1200), from Old French proceder "to proceed" (see proceed). Meaning "method of conducting business in Parliament" is from 1839.
proceed (v.): late 14c., "to go on," also "to emanate from, result from," from Old French proceder (13c., Modern French procéder) and directly from Latin procedere (past participle processus) "go before, go forward, advance, make progress; come forward," from pro- "forward" (see pro-) + cedere "to go" (see cede). Related: Proceeded; proceeding.
proceeding (n.): 1510s, "action of going forward," verbal noun from proceed (v.). From 1550s as "what is done, conduct." Proceedings "records of the doings of a society" is attested by 1824.
proceedings (n.): "records of the doings of a society," by 1824; see proceeding.
proceeds (n.): "results, profits," 1660s, from proceed (v.) on the notion of "that which proceeds" from some event or activity.
procerity (n.): "tallness," 1540s, from Latin proceritas, from procerus "high, tall," from pro- (see pro-) + -cerus, from stem of crescere "to grow" (see crescent).
process (n.): early 14c., "fact of being carried on" (as in in process), from Old French proces "a journey; continuation, development; legal trial" (13c.) and directly from Latin processus "a going forward, advance, progress," from past participle stem of procedere "go forward" (see proceed).

Meaning "course or method of action" is from mid-14c.; sense of "continuous series of actions meant to accomplish some result" (the main modern sense) is from 1620s. Legal sense of "course of action of a suit at law" is attested from early 14c.
process (v.1): 1530s, "begin legal action against," from Middle French processer "to prosecute," from proces (see process (n.)). Meaning "prepare by special process" is from 1881, from the noun in English. Of persons, "to register and examine," by 1935. Related: Processed; processing.
process (v.2): "to go in procession," 1814, "A colloquial or humorous back-formation" from procession [OED]. Accent on second syllable.
procession (n.): late Old English, "act of marching or proceeding," from Old French procession "procession" (religious or secular), 11c., and directly from Late Latin processionem (nominative processio) "religious procession," in classical Latin "a marching onward, a going forward, advance," noun of action from past participle stem of procedere (see proceed).
processional (n.): "book of hymns for use in processions," mid-15c., from Medieval Latin processionale, from noun use of neuter of processionalis "pertaining to a procession," from Late Latin processio (see procession).
processional (adj.): "pertaining to a procession or processions," 1610s, from procession (n.) + -al (1).
processor (n.): 1909, agent noun in Latin form from process (v.). Data processor is from 1957; word processor is from 1973; food processor in the kitchen appliance sense also is from 1973.
proclaim (v.): late 14c., proclamen, from Latin proclamare "cry or call out," from pro- "forth" (see pro-) + clamare "to cry out" (see claim (v.)). Spelling altered by influence of claim. Related: Proclaimed; proclaiming; proclaimer.
proclamation (n.): late 14c., "act of making public," also "that which is proclaimed;" from Old French proclamacion (14c., Modern French proclamation) and directly from Latin proclamationem (nominative proclamatio), noun of action from past participle stem of proclamare (see proclaim).
proclitic (adj.): 1846, from Medieval Latin procliticus, formed on analogy of encliticus from Greek proklinein "to lean forward," from pro- "forward" (see pro-) + klinein "to lean" (see lean (v.)).
proclivity (n.): 1590s, from Middle French proclivité or directly from Latin proclivitatem (nominative proclivitas) "a tendency, predisposition, propensity," from proclivis "prone to," literally "sloping, inclined," from pro- "forward" (see pro-) + clivus "a slope," from PIE *klei-wo-, suffixed form of *klei "to lean" (see lean (v.)).
proconsul (n.): late 14c., "governor or military commander of an ancient Roman province," from Latin proconsul "governor of a province; military commander," from phrase pro consule "(acting) in place of a consul," from pro- "in place of" (see pro-) + ablative of consul. In modern use usually rhetorical, but it was a title of certain commissioners in the French Revolution, was used in English for "deputy consul," and was used again of U.S. administrators in Iraq during the occupation. Related: Proconsular.
procrastinate (v.): 1580s, a back formation from procrastination or else from Latin procrastinatus, past participle of procrastinare "to put off till tomorrow; defer, delay" (see procrastination). Related: Procrastinated; procrastinating. Earlier verb was procrastine (1540s), from French.
Do not put off till tomorrow what can be put off till day-after-tomorrow just as well. [Mark Twain]
procrastination (n.): 1540s, from Middle French procrastination and directly from Latin procrastinationem (nominative procrastinatio) "a putting off from day to day," noun of action from past participle stem of procrastinare "put off till tomorrow, defer, delay," from pro- "forward" (see pro-) + crastinus "belonging to tomorrow," from cras "tomorrow," of unknown origin.
procrastinator (n.): c. 1600, agent noun in Latin form from procrastinate (v.).
procreant (adj.): "fruitful," 1580s, from Latin procreantem (nominative procreans), present participle of procreare "to beget" (see procreation). As a noun from c. 1600.
procreate (v.): 1530s, a back formation from procreation or else from Latin procreatus, past participle of procreare "to beget, bring forth" (see procreation). Related: Procreated; procreating.