"Fuck off"

My brother walked in on me once. And by that I mean he went and found my parents emergency spare key to my room and barged in demanding his turn with the modem.

I spun around, cock in hand, and said, "Fuck off, I'm having a wank", and my dad shouted up the stairs, "Leave your brother alone when he's floggin' it".

He never barged in on me again.