Green’s Dictionary of Slang

bun n.1

[? 16C north. dial. bun, the tail of a hare; also abbr. SE bunny, the stereotypically ‘sexy’ rabbit]

1. [late 16C–19C] a squirrel.

2. [late 18C–19C] a rabbit.

3. referring to the stereotyped sexual appetites of sense 2.

(a) [mid-17C+] pubic hair; thus the vagina.

(b) [1930s+] (Scot.) a prostitute.

4. [late 19C+] an attractive female who is ‘good enough to eat’.

In phrases

butter a bun (v.)

[1950s+] (US) to have sexual intercourse.

touch the bun for luck (v.) [orig. a RN tradition]

[late 18C–early 19C] to touch one’s wife’s or girlfriend’s genital area for luck before leaving on a journey.