Green’s Dictionary of Slang

swap v.

also swop
[echoic swap, the sound of a slap; thus the slaps exchanged on sealing a bargain. Orig. Irish tinker’s/horse-dealer’s term, to swap a bargain, to strike a deal. Despite these origins, sense 1 is now effectively SE]

1. [late 16C+] to exchange.

2. [mid-19C–1900s] to dismiss from a job.

3. [late 19C] (US, also swap off) to cheat, to take in.

In phrases

swap cans (v.) [can n.1 (1b)]

[1940s+] (US prison) to take alternate active/passive roles in anal intercourse.

swap dandruff (v.)

[1930s] to talk in confidence.

swap gravy (v.) [gravy n. (1)]

[2000s] (US black) to have sexual intercourse.

swap lead (v.)

[1940s] to engage in a shooting match.

swap off

see sense 3 above.

swap out

see separate entries.

swap spit(s) (v.)

1. [1930s+] to perform oral intercourse.

2. [1940s+] (also suck spit, swap chews, ...tongues) to kiss, usu. with tongues.

3. [1970s+] in joc./fig. use, to become intimate, to bond.