A sissy wee is when you pee in your knickers, but just a little bit.
not enough to show, but enough to get nervous and to know you have a weak piss-spot.
Ginnie - "Oh fuck, ive just had a sissy wee in my panties! but just a bit. i have such a loose pissing hole."
Jools - "dont worry. you are wearing your large bloomers."