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 (the title of a 20th Century Fox film starring June Haver).
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.
1570s, "customer," short for obsolete chapman in its secondary sense "purchaser, trader" (also see cheap). Colloquial familiar sense of "lad, fellow, man or boy" is first attested 1716, usually with a qualifying adjective. Compare slang (tough) customer and German Kunde "customer, purchaser," colloquially "fellow."
"connoisseur in eating and drinking," 1820, from French gourmet, altered (by influence of gourmant "glutton") from Old French groume, originally "wine-taster, wine merchant's servant" (in 13c. "a lad generally"), a word of uncertain origin. As an adjective from 1900. Compare groom (n.1). For sense distinction, see gourmand.
"youth, lad; boy of the lower orders; personal servant," c. 1300 (early 13c. as a surname), originally also "youth preparing to be a knight" (beneath the rank of a squire), from Old French page "a youth, page, servant" (13c.), possibly via Italian paggio (Barnhart), from Medieval Latin pagius "servant," perhaps ultimately from Greek paidion "boy, lad," diminutive of pais (genitive paidos) "child."
But OED considers this unlikely and, with Century Dictionary, points instead to Littré's suggestion of a source in Latin pagus "countryside," in sense of "boy from the rural regions" (see pagan). Meaning "youth employed as a personal attendant to a person of rank" is first recorded mid-15c.; this was transferred from late 18c. to boys who did personal errands in hotels, clubs, etc., also in U.S. legislatures.