Word Origin & History
late 13c., gyrle "child" (of either sex), of unknown origin; current scholarship leans toward an unrecorded O.E. *gyrele, from P.Gmc. *gurwilon-, dim. of *gurwjoz (represented by Low Ger. gære "boy, girl"), from PIE *ghwrgh-, also found in Gk. parthenos "virgin." But this is highly conjectural.
Another candidate is O.E. gierela "garment." Like boy, lass, lad it is of 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 14c. Applied to "any young unmarried woman" since 1520s. Meaning "sweetheart" is from 1648. Girl next door as a type of unflashy attractiveness is first recorded 1961.