Green’s Dictionary of Slang

1. the vagina.

1974

D. Mamet Sexual Perversity in Chicago (1994) 84: Do you want me walking around with a naked la-la?

2. (W.I.) a sexually permissive girl.

1983

T. White Catch a Fire 137: Young rude bwais [...] make the ‘la-la’ (sexually indiscriminate) girls at blues want to wriggle.