1810, American English variant of mamma (q.v.). Apparently first used in the South and with a racial context. As a biker's girlfriend or female passenger, from 1950s.
An old negro woman is called momma, which is a broad pronunciation of mama ; and a girl, missy. I once happened to call a young negro wench momma—"me be no momma," says she, "me had no children yet." [John Lambert, "Travels through Lower Canada, and the United States of North America in the Years 1806, 1807, and 1808," London, 1810]