Green’s Dictionary of Slang

1. (US prison) to indulge in gay sex.

1977		E. Bunker Animal Factory 86: After ten years ‘making tortillas’ or ‘flip-flopping’ was acceptable.
1981		Maledicta V:1+2 (Summer + Winter) 265: A homosexual may flip-flop with a fellow bitch by reciprocating sexually.
2001		Gay Video Dad Behind Closed Doors 🌐 Rev. of Natural Instinct: Cummings and Grand flip-flop so Grand can toss some salad and then use his meat stick to poke at Cummings’ hot ass.

2. to change direction.

1980		in S. Terkel Amer. Dreams (1982) 373: It wasn’t until I saw this guy flip-flop that I realized how powerful vested interests are. Suddenly he’s saying: ‘Don’t rock the boat’.
1997		Mad mag. Sept. 27: Now he flip-flops in the breeze / Sucking up to the Chinese.
2017		T. Spicer Good Girl Stripped Bare 244: ‘Choose a side and stay with it. Don’t flip-flop’.

3. (US prison) of a male homosexual, to take either the active or passive role in intercourse.

1992		Bentley & Corbett Prison Sl. 59: Flip Flop […] used to indicate a homosexual who engages in both the active and passive roles in a homosexual relationship. A homosexual who flip flops will ‘pitch’ and ‘catch.’.
1996		L. Pettiway Honey, Honey, Miss Thang 27: I don’t know if I could be in a relationship with a man who likes to flip flop.

4. (US prison) to practise bisexuality.

1992

Bentley & Corbett Prison Sl. 59: Flip Flop To have sex with both men and women. A bisexual person is said to flip flop.