In an online gaming lobby, to be told to ready up is when someone is ready to start a game and you must press a corresponding button to be 'ready'.
gamer 1: hey guys ready up
gamer 2: but i'm not ready
A cool slang way of sayin your ready. Popularly used in North West LondonNorth Weezy
"Safe bruv u ready up?"
"Yea yea im ready straight, lets roll"