Green’s Dictionary of Slang

fanny n.1

[ety. unknown. The suggestion in DSUE and elsewhere of a link to Fanny Hill, the heroine of John Cleland’s Memoirs of a Woman of Pleasure (1749) does not stand; see P. Spedding and J. Lambert ‘Fanny Hill, Lord Fanny, and the Myth of Metonymy’ in Studies in Philology 108, No. 1 (Winter, 2011), pp. 108-132 ]

1. [mid-19C+] (UK only) the vagina.

2. [1910s] (US, also fannie) an old(er) woman.

3. [1910s+] (US, also fannie, fannyolo) the buttocks; thus fanny-shaker, a bellydancer.

4. [1920s+] (US) one’s self.

5. [1990s+] women, considered simply as sex objects.

6. [2000s] a fool, an idiot, a general term of abuse.

In compounds

fanny-artful (n.) (also fanny-fair)

[19C] the vagina.

fanny batter (n.)

[1990s+] vaginal secretions.

fanny-baws (n.)

[2010s] (Scot.) a weakling, an effeminate man.

fanny flaps (n.)

[1990s+] the labia; thus fanny-flapped, a derog. epithet for a woman.

fanny-head (n.)

[2010s] (Scot.) a fool.

fanny merchant (n.)

[1980s+] a womaniser.

fanny nudger (n.)

[1990s+] a vibrator.

fanny pack (n.) (also fanny flask)

[1950s+] (US) a small pouch-like bag strapped around the wearer’s waist.

fanny pad (n.)

[2000s+] a sanitary pad.

fanny rag (n.)

[1940s+] (Aus.) a sanitary towel.

fanny rat (n.)

1. [1940s–50s] a pubic louse.

2. [1990s+] the penis.

3. [1990s+] a promiscuous man.

4. [1990s+] a general term of abuse.

5. [2000s] a womanizer.

In phrases

shake one’s fanny (v.)

1. [1930s] (orig. US) to hurry up.

2. to have sexual intercourse.

In exclamations

my fanny! (also my old fanny! my sister’s fanny!)

[late 17C+] a general excl. of disdain, dismissal, arrogant contempt.