tallboy n.

"high in stature," 1520s, probably from Middle English tal "handsome, good-looking; valiant; lively in speech; large, big; humble, meek," from Old English getæl "prompt, active," from Germanic *(ge)-tala- (source also of Old High German gi-zal "quick," Gothic un-tals "indocile"). Main modern sense "being of more than average height (and slim in proportion to height)" probably evolved out of earlier meanings "brave, valiant, seemly, proper" (c. 1400), "attractive, handsome" (late 14c.).

Sense evolution is "remarkable" [OED], but adjectives applied to persons can wander far in meaning (such as pretty, buxom, German klein "small, little," which in Middle High German meant the same as its English cognate clean (adj.)). Meaning "having a (defined) height," whether lofty or not is from 1580s. Meaning "exaggerated" (as in tall tale) is American English colloquial attested by 1846. Phrase tall, dark, and handsome is recorded from 1906. Related: Tallness.

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]

also high-boy, "tall chest of drawers," 1891, American English (see tallboy); a hybrid, the second element is from French bois "wood" (see bush (n.)).