Although "fud" is widely accepted in Scotland as being a slang term for the female reproductive organs, it is generally used as a pejorative to describe someone who has just done something stupid, often in situations where they've either been impulsive or it was blindingly obvious beforehand that it was a stupid thing to do. It can also be used to describe someone who irritates everyone because they try to impress everyone all the time & invariably talk a load of pish.
"Haw ya big fud!" (angry)
Person A "Ah canny be fucked wi him!"
Person B "How?"
Person A "Coz he's a pure fud!"