Online Etymology Dictionary

long (v.): Old English langian "to yearn after, grieve for," literally "to grow long, lengthen," from Proto-Germanic *langojanan (see long (adj.)). Cognate with Old Norse langa, Old Saxon langon, Middle Dutch langhen, Old High German langen "to long," German verlangen "to desire." Related: Longed; longing.
long johns (n.): type of warm underwear, 1943, originally for U.S. GIs. By 1919 as a type of pastry. Long john also was used of various sorts of worm, potato, sled, etc.
long pig (n.): "human being eaten as food," 1848, in a Pacific Islander context:
Bau literally stank for many days, human flesh having been cooked in every house, and the entrails thrown outside as food for pigs, or left to putrefy in the sun. The Somosomo people were fed with human flesh during their stay at Bau, they being on a visit at that time; and some of the Chiefs of other towns, when bringing their food, carried a cooked human being on one shoulder, and a pig on the other; but they always preferred the "long pig," as they call a man when baked. ["FEEJEE.--Extract of a Letter from the Rev. John Watsford, dated Ono, October 6th, 1846." in "Wesleyan Missionary Notices," Sept. 1847]
long run (n.): also long-run, "ultimate outcome," 1620s, from long (adj.) + run (n.), on notion of "when events have run their course." As an adjective from 1804.
long shot (n.): in the figurative sense of "something unlikely," 1867, from long (adj.) + shot (n.). The notion is of a shot at a target from a great distance, thus difficult to make. Cinematic sense is from 1922.
long-ago (adj.): 1834, from long (adj.) + ago.
long-distance (adj.): 1884, in reference to telephoning, from long (adj.) + distance (n.).
long-headed (adj.): "discerning," c. 1700, from long (adj.) + head (n.).
long-lived (adj.): early 15c., from long (adj.) + past participle of live (v.). Old English had langlife "long-lived."
long-playing (adj.): 1910, of recordings, from long (adj.) + present participle of play (v.).
long-running (adj.): 1943, of theatrical productions, from long (adj.) + running.
long-suffering: also longsuffering, 1520s (n.), 1530s (adj.), from long (adj.) + suffering (see suffer). Old English had langmodig in this sense.
long-term (adj.): also longterm, long term, 1876, originally in insurance, from long (adj.) + term (n.).
long-winded (adj.): also longwinded, 1580s, "given to lengthy speeches," from long (adj.) + adjective from wind (n.1) in the secondary Middle English sense "breath in speaking" (early 14c.).
longanimity (n.): "patience," mid-15c., from Late Latin longanimitas "long-suffering, patient," from longanimus, from longus (see long (adj.)) + animus "soul, spirit, mind" (see animus).
longbow (n.): also long-bow, the characteristic medieval English weapon, c. 1500, from long (adj.) + bow (n.1).
longeron (n.): 1912, from French longeron, from longer "to skirt, extend along," from allonger "to lengthen" (see lunge).
longevity (n.): 1610s, from Late Latin longaevitatem (nominative longaevitas) "great age, long life," from Latin longaevus "long-lived," from longus (see long (adj.)) + aevum "lifetime, age" (see eon).
longhair (n.): also long-hair, "cat with long hair," 1893, from long (adj.) + hair. As "intellectual," especially in musical tastes, "devotee of classical music," 1920. Sense of "hippie" attested from 1969.
longhand (adj.): also long-hand, of handwriting, 1660s, from long (adj.) + hand (n.).
longhorn (adj.): also long-horn, in reference to a type of cattle, 1808, from long (adj.) + horn (n.).
longi-: word-forming element meaning "long," from Latin longi-, comb. form of longus (see long (adj.)).
longing (n.): "yearning, desire," Old English langung "longing, weariness, sadness, dejection," from long (v.). Related: Longingly.
longinquity (n.): "remoteness," 1540s, from Latin longinquitas "length, extent, duration," from longinquus "long, extensive, remote, distant," from longus (see long (adj.)) -inquus.
longish (adj.): 1610s, from long (adj.) + -ish.
longitude (n.): late 14c., "length," from Latin longitudo "length, duration," from longus (see long (adj.)). For origins, see latitude.
longitudinal (adj.): 1706, from Latin longitudo (see longitude) + -al (1).
longship (n.): Old English langscip "man of war;" see long (adj.) + ship (n.).
longshoreman (n.): 1811, shortening of alongshore + man (n.).
longstanding: also long-standing, c. 1600 (n.), 1814 (adj.), from long (adj.) + standing.
longtime (adj.): also long-time, 1580s, from long (adj.) + time (n.).
longways (adv.): 1580s, from long (adj.) + way (n.) + adverbial genitive -s.
loo (n.1): "lavatory," 1940, but perhaps 1922, probably from French lieux d'aisances, "lavatory," literally "place of ease," picked up by British servicemen in France during World War I. Or possibly a pun on Waterloo, based on water closet.
loo (n.2): type of card game, 1670s, short for lanterloo (1660s), from French lanturelu, originally (1620s) the refrain of a popular comic song; according to French sources the refrain expresses a mocking refusal or an evasive answer and was formed on the older word for a type of song chorus, turelure; apparently a jingling reduplication of loure "bagpipe" (perhaps from Latin lura "bag, purse").
From its primary signification -- a kind of bagpipe inflated from the mouth -- the word 'loure' came to mean an old dance, in slower rhythm than the gigue, generally in 6-4 time. As this was danced to the nasal tones of the 'loure,' the term 'loure' was gradually applied to any passage meant to be played in the style of the old bagpipe airs. ["Grove's Dictionary of Music and Musicians," London, 1906]
The refrain sometimes is met in English as turra-lurra.
looey (n.): 1916, American English, colloquial familiar form of lieutenant.
loof (n.): "palm of the hand," Scottish and Northern English, c. 1300, from Old Norse lofe, cognate with Gothic lofa, Russian lapa "paw," Lettish lepa "paw."
loofah (n.): 1879, from Egyptian Arabic lufah, the name of the plant (Luffa ægyptiaca) with fibrous pods from which flesh-brushes are made.
loogie (n.): "nasal mucus," U.S. slang, by 1990.
look (v.): Old English locian "use the eyes for seeing, gaze, look, behold, spy," from West Germanic *lokjan (source also of Old Saxon lokon "see, look, spy," Middle Dutch loeken "to look," Old High German luogen, German dialectal lugen "to look out"), of unknown origin, perhaps cognate with Breton lagud "eye." In Old English, usually with on; the use of at began 14c. Meaning "seek, search out" is c. 1300; meaning "to have a certain appearance" is from c. 1400. Of objects, "to face in a certain direction," late 14c.

Look after "take care of" is from late 14c., earlier "to seek" (c. 1300), "to look toward" (c. 1200). Look into "investigate" is from 1580s; look up "research in books or papers" is from 1690s. To look down upon in the figurative sense is from 1711; to look down one's nose is from 1921. To look forward "anticipate" is c. 1600; meaning "anticipate with pleasure" is mid-19c. To not look back "make no pauses" is colloquial, first attested 1893. In look sharp (1711) sharp originally was an adverb, "sharply."
look (n.): c. 1200, "act or action of looking," from look (v.). Meaning "appearance of a person" is from late 14c. Expression if looks could kill ... attested by 1827 (if looks could bite is attested from 1747).
look-alike (n.): "someone who closely resembles another," 1937, American English, from look (v.) + alike.
look-down (n.): type of sea fish, 1882, from look (v.) + down (adv.). So called from facial structure.
look-see (n.): "inspection," 1865, "Pidgin-like formation" [OED], and first used in representations of English as spoken by Chinese, from look (v.) + see (v.).
looker (n.): Old English locere "one engaged in looking," agent noun from look (v.). Meaning "one who watches over" is from c. 1300; that of "one who has a certain appearance" is late 15c. Slang meaning "attractive woman" attested from 1893; good-looker is attested from 1866 of both women and horses. Looker-in (1927) was an early word for "television viewer."
looking-glass (n.): 1520s, from looking, present participle adjective from look (v.) + glass (n.).
lookout (n.): also look-out, "person who stands watch or acts as a scout," 1690s, from look + out. Verbal phrase look out "be on the watch" attested from c. 1600.
loom (n.): weaving machine, Old English geloma "utensil, tool," from ge-, perfective prefix, + -loma, of unknown origin (compare Old English andloman (plural) "apparatus, furniture"). Originally "implement or tool of any kind" (as in heirloom); thus, "the penis" (c. 1400-1600). Specific meaning "a machine in which yarn or thread is woven into fabric" is from c. 1400.
loom (v.): 1540s, "to come into view largely and indistinctly," perhaps from a Scandinavian source (compare dialectal Swedish loma, East Frisian lomen "move slowly"), perhaps a variant from the root of lame (adj.). Early used also of ships moving up and down. Figurative use from 1590s. Related: Loomed; looming.
loon (n.1): large diving bird (especially the Great Northern Diver), 1630s, from a Scandinavian source (compare Norwegian lom, from Old Norse lomr).
loon (n.2): "crazy person," mid-15c., lowen "rascal," of uncertain origin, perhaps related to Dutch loen "stupid person."