"young male enthusiast," by 1988, from fan (n.2) + boy. Fangirl attested from 1989.
Entries linking to fanboy
fan n.2
"devotee," 1889, American English, originally of baseball enthusiasts, probably a shortening of fanatic, but it may be influenced by the fancy, a collective term for followers of a certain hobby or sport (especially boxing); see fancy (n.). There is an isolated use from 1682, but the modern word likely is a late 19c. formation. Fan mail attested from 1920, in a Hollywood context; Fan club attested by 1930.
Before the close of the republic, an enthusiastic partisan of one of the factions in the chariot races flung himself upon the pile on which the body of a favourite coachman was consumed, and perished in the flames. [W.E.H. Lecky, "A History of European Morals," 1869]
boy n.
mid-13c., boie "servant, commoner, knave" (generally young and male); c. 1300, "rascal, ruffian, knave; urchin," mid-14c. as "male child before puberty" (possibly extended from the "urchin" sense). A word of unknown origin.
Possibly from Old French embuie "one fettered," from Vulgar Latin *imboiare, from Latin boia "leg iron, yoke, leather collar," from Greek boeiai dorai "ox hides." (Words for "boy" double as "servant, attendant" across the Indo-European map — compare Italian ragazzo, French garçon, Greek pais, Middle English knave, Old Church Slavonic otroku — and often it is difficult to say which meaning came first.)
But it also appears to be identical with East Frisian boi "young gentleman," and perhaps with Dutch boef "knave," from Middle Dutch boeve, perhaps from Middle Low German buobe. This suggests a gradational relationship to babe. Another conjecture:
In Old English, only the proper name Boia has been recorded. ME boi meant 'churl, servant' and (rarely) 'devil.' In texts, the meaning 'male child' does not antedate 1400. ModE boy looks like a semantic blend of an onomatopoeic word for an evil spirit (*boi) and a baby word for 'brother' (*bo). [Liberman]
Used slightingly of young men in Middle English, also in familiar or contemptuous use of criminal toughs or men in the armed services. In some local uses "a man," without reference to age (OED lists "in Cornwall, in Ireland, in the far West of the U.S."). The meaning "male negro slave or Asian personal servant of any age" attested from c. 1600.
Extended form boyo is attested from 1870. Emphatic exclamation oh, boy is attested by 1917. Boy-meets-girl "typical of a conventional romance" is from 1945; the phrase itself is from 1934 as a dramatic formula. Boy-crazy "eager to associate with males" is from 1923.
A noticable number of the modern words for 'boy', 'girl', and 'child' were originally colloquial nicknames, derogatory or whimsical, in part endearing, and finally commonplace. These, as is natural, are of the most diverse, and in part obscure, origin. [Buck]