A girl who is relatively old or mature; (also) a girl who is old enough to look after herself or to cope with adult experiences (also (informal) used ironically, of a woman).
Origin
Mid 17th century. From big + girl. Compare earlier big boy.
Definition of big girl in US English:
big girl
nounˈbɪɡ ˌɡəːl
A girl who is relatively old or mature; (also) a girl who is old enough to look after herself or to cope with adult experiences (also (informal) used ironically, of a woman).
Origin
Mid 17th century. From big + girl. Compare earlier big boy.