Scranny

In the phrase: Bob's Your Uncle, Fanny's your Aunt where does the Fanny's your Aunt come from?

Also what is the origin of the term scranny. It was often used to describe a girl when I was at school?
i.e. a nice bit of scranny..... someone has told me that it could be a old english word for a tasty morsel
of food... Any other info?

Scran"ny) a. [See Scrannel.] Thin; lean; meager; scrawny; scrannel. [Prov. Eng. & Scot.] Lean, thin, bony, lank, meagre, gaunt, angular, skinny, emaciated, attenuated, scranny, scrawny.