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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
by TH3 R1G July 09, 2011
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"
by Al Pacino NW October 11, 2006