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girl: girl: [13] Where girl comes from is one of the unsolved puzzles of English etymology. What is at least clear is that originally it meant ‘child’ in general rather than specifically ‘female child’ (a mid 15th-century text refers to knave-gerlys ‘male children’), but where it came from is not known. Among suggestions for words that may be connected are Low German göre ‘child, kid’ and Norwegian dialect gurre ‘lamb’.
boy (n.): mid-13c., boie "servant, commoner, knave, boy," 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. For a different 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]

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]
Used slightingly of young men in Middle English; meaning "male negro slave or Asian personal servant of any age" attested from c. 1600. Exclamation oh, boy attested from 1892.
girl (n.): c. 1300, gyrle "child, young person" (of either sex but most frequently of females), of unknown origin. One guess [OED] leans toward an unrecorded Old English *gyrele, from Proto-Germanic *gurwilon-, diminutive of *gurwjoz (apparently also represented by Low German gære "boy, girl," Norwegian dialectal gorre, Swedish dialectal gurre "small child," though the exact relationship, if any, between all these is obscure), from PIE *ghwrgh-, also found in Greek parthenos "virgin." But this involves some objectionable philology. Liberman (2008) writes:
Girl does not go back to any Old English or Old Germanic form. It is part of a large group of Germanic words whose root begins with a g or k and ends in r. The final consonant in girl is a diminutive suffix. The g-r words denote young animals, children, and all kinds of creatures considered immature, worthless, or past their prime.
Another candidate is Old English gierela "garment" (for possible sense evolution in this theory, compare brat). A former folk-etymology derivation from Latin garrulus "chattering, talkative" is now discarded. Like boy, lass, lad it is of more or less obscure origin. "Probably most of them arose as jocular transferred uses of words that had originally different meaning" [OED]. Specific meaning of "female child" is late 14c. Applied to "any young unmarried woman" since mid-15c. Meaning "sweetheart" is from 1640s. Old girl in reference to a woman of any age is recorded from 1826. Girl next door as a type of unflashy attractiveness is recorded by 1953.
Doris [Day] was a big vocalist even before she hit the movies in 1948. There, as the latest movie colony "girl next door," sunny-faced Doris soon became a leading movie attraction as well as the world's top female recording star. "She's the girl next door, all right," said one Hollywood admirer. "Next door to the bank." ["Life" magazine, Dec. 22, 1958]
Girl Friday "resourceful young woman assistant" is from 1940, a reference to "Robinson Crusoe." Girl Scout is from 1909. For the usual Old English word, see maiden.
godson (n.): "male child one sponsors at baptism," c. 1200, from God + son, replacing or modifying Old English godsunu.
groom (n.1): c. 1200 (late 12c. in surnames), grome "male child, boy;" c. 1300, "a youth, young man," also "male servant, attendant, minor officer in a royal or noble household ranking higher than a page; a knight's squire." Of unknown origin; no certain cognates in other Germanic languages. Perhaps from an unrecorded Old English *grom, *groma, which could be related to growan "to grow," and influenced by guma "man." Or perhaps from or influenced by Old French grommet "boy, young man in service, serving-man" (compare Middle English gromet "ship's boy," early 13c.). As the title of an officer of the English royal house from mid-15c. Specific meaning "male servant who attends to horses and stables" is from 1660s, from earlier combinations such as horse-groom, Groom of the Stables, etc.
manchild (n.): also man-child, "male child, male infant," c. 1400, from man (n.) + child.