Oh dear, I needed my eye-plugs for that swearing/slang site
Strange ... here a "fanny' is a bottom/bum/heinie.
That's not what it is there ...
Flashback to being a young teenager living with my English (Bristol born?) foster family and the first times I heard them use two expressions that totally shocked me coming out of such nice and very proper people ...
"Keep your pecker up"
"I'll come knock you up in the morning"
I knew what those references meant HERE but not what they meant THERE

I'll never forget that!