"companion, comrade," c. 1200, from Old English feolaga "partner, one who shares with another," from Old Norse felagi, from fe "money" (see fee) + lag, from Proto-Germanic *lagam, from PIE root *legh- "to lie down, lay." The etymological sense of fellow seems to be "one who puts down money with another in a joint venture."
Meaning "one of the same kind" is from early 13c.; that of "one of a pair" is from c. 1300. Used familiarly since mid-15c. for "any man, male person," but not etymologically masculine (it is used of women, for example, in Judges xi.37 in the King James version: "And she said unto her father, Let this thing be done for me: let me alone two months, that I may go up and down upon the mountains, and bewail my virginity, I and my fellows").
Its use can be contemptuous or dignified in English and American English, and at different times in its history, depending on who used it to whom, it has carried a tinge of condescension or insult.
University senses (mid-15c., corresponding to Latin socius) evolved from notion of "one of the corporation who constitute a college" and who are paid from its revenues. Fellow well-met "boon companion" is from 1580s, hence hail-fellow-well-met as a figurative phrase for "on intimate terms."
In compounds, with a sense of "co-, joint-," from 16c., and by 19c. also denoting "association with another." Hence fellow-traveler, 1610s in a literal sense but in 20c. with a specific extended sense of "one who sympathizes with the Communist movement but is not a party member" (1936, translating Russian poputchik).
Fellow-countrymen formerly was one of the phrases the British held up to mock the Americans for their ignorance, as it is redundant to say both, until they discovered it dates from the 1580s and was used by Byron and others.
an attempt at a phonological spelling of a casual pronunciation of fellow (n.), attested by 1864 (as fellah). Feller, along the same lines, is recorded by 1825. Earlier, Pope rhymes fellow with prunella ("Essay on Man," epistle IV).
c. 1200, feolahschipe "companionship," from fellow + -ship. Sense of "a body of companions" is from late 13c. Meaning "spirit of comradeship, friendliness" is from late 14c. As a state of privilege in English colleges, from 1530s. In Middle English it was at times a euphemism for "sexual intercourse" (carnal fellowship).
To fellowship with is to hold communion with; to unite with in doctrine and discipline. This barbarism now appears with disgusting frequency in the reports of ecclesiastical conventions, and in the religious newspapers generally. [Bartlett, "Dictionary of Americanisms," 1848]
But Chaucer and Wyclif used it as a verb in Middle English, "to have fellowship with."
It forms all or part of: allay; anlage; belay; beleaguer; bylaw; coverlet; fellow; lager; lair; law; lawful; lawless; lawsuit; lawyer; lay (v.) "to cause to lie or rest;" ledge; ledger; lees; lie (v.2) "rest horizontally;" litter; lochia; low (adj.) "not high;" outlaw; scofflaw; stalag; vorlage.
It is the hypothetical source of/evidence for its existence is provided by: Hittite laggari "falls, lies;" Greek lekhesthai "to lie down," legos "bed," lokhos "lying in wait, ambush," alokhos "bedfellow, wife;" Latin lectus "bed;" Old Church Slavonic lego "to lie down;" Lithuanian at-lagai "fallow land;" Old Irish laigim "I lie down," Irish luighe "couch, grave;" Old English licgan "be situated, have a specific position; remain; be at rest, lie down."
"insolent person, boisterous fellow," 1560s, a mock surname from rude + -by, common ending element in place-names (and thus surnames), as in Grimsby, Rigby, Catesby. Similar formations in idlesby "lazy fellow" (1610s), sneaksby "paltry, sneaking fellow" (1570s), suresby (16c.), lewdsby (1590s), nimblesby (1610s).