[Opinions] here in the US ...
Fanny is slang for backside, butt. It isn't vulgar like in the UK, more like something your grandma might say, and never in a sexualized way. "Sit your fanny down and be quiet" or "I fell down flat on my fanny" for instance.
We don't need to set the sky on fire. A little glow will do just fine.
Bob Ross