Green’s Dictionary of Slang

fanny n.2

[? fig. use of fanny n.1 (3) on model of ballocks n. (4)]

1. [1910s+] verbal effusiveness, usu. nonsensical.

2. [1920s+] any form of story (poss. mendacious) designed to elicit money or sympathy, to provide excuses etc.

3. in non-verbal sense, nonsense, absurdity.

4. [2000s] a fit of temper.

In compounds

fanny merchant (n.) [merchant n. (1)]

[1990s+] one who offers only empty boasts and promises.

fanny mob (n.) [mob n.2 (3)]

[1970s] (UK Und.) confidence tricksters.

In phrases

spin a fanny (v.)

[1930s–50s] to tell a deceitful story.