Green’s Dictionary of Slang

mommy n.

1. (S.Afr./US gay) the lesbian equivalent of a daddy n. (7)

[US]R. Giallombardo Gloss. in Study of a Women’s Prison 124: The ‘femme’ or ‘mommy’ plays the female role in a homosexual relationship.
[SA]K. Cage Gayle.

2. see momma n.1 (1)