"In Middle English, male children were 'knave' girls while female children were 'gay' girls."

"Only in Modern English did 'girl' come to refer only to female children. Why this happened isn’t certain, but it’s not uncommon for words to specialize over time in that way: 'meat' originally referred to all food or sustenance, kind of like the phrase 'daily bread'; 'apple' to fruits generally; and 'queen' began as a word for 'woman' in general, and only later specialized royally. However, which words go in which directions can be quite fortuitous, and some words acquire wider, rather than narrower meanings. 'Dog' first referred to certain larger, more powerful dogs, then came to mean all dogs. 'Child' came to mean all pre-adults rather than just the littlest ones. As we moved from Middle English to Modern English, even 'child' could take on a female meaning in a certain context. A line in Shakespeare’s 'The Winter’s Tale' asks, 'A boy or a child, I wonder?' And one might suppose that this suggests that a girl is somehow less distinctive than a boy, a mere generic 'child.' But it’s possible this actually traces back to when 'child' referred specifically to female children."

Writes John McWhorter in "'Woman' and 'Female' Didn’t Start as Words About Men" (NYT).