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"You know what you doing" first appeared in a game for the Sega Genesis called "Zero Wing", along with the phrases All your base are belong to us and You have no chance to survive, make your time.

One uses this phrase to point out that the listener is or the listeners are capable of finishing some task.
Alexander: I'm going to call the guy, and convince him that we are right about this.
9001: You know what you doing!
Alexander: Thanks for the encouragement.
by 9001 June 07, 2010
taken from the zero wing dialect (see all your base are belong to us). stands for you know what you are doing, or we're counting on you. Usually achieved by launching all zig for great justice, when we have no chance to survive and are making our time.
"I know what im doing!"
"you know what you doing! launch all zig, for great justice!"
by baklava March 14, 2004