Green’s Dictionary of Slang

home run n.

[baseball imagery]

1. [1900s] an escape from prison.

2. [1960s+] (US, also homer) sexual intercourse; both heterosexual and homosexual (cf. first base n. (1); second base under second adj.; third base under third adj.).

In phrases

hit a home run (v.) (also score a home run, get run)

[1980s+] (US campus) to have sexual intercourse.