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1.
While drunk, trying to act sober in front of the person your trying to hook up with, as well as commanding your friends who are even more drunk to act like they're sober. Originated in Athens, Greece.
Girl 1: I see a hot guy over there.
Girl 2: You better be captain sober if you want to get him.

or

Friend 1: Your acting ridiculously drunk right now, you need to be captain sober.
by Captain Greece! March 29, 2011
 
2.
When a single man is sober and takes charge in a group of miscreants that are drunk
Adam - Oh no! The teacher is gunna see us all drunk
Captain Sober - Don't worry I got this guys. Go hide over there.
Teacher - Whats going on here?
Captain Sober - Nothing everything is perfectly fine, I mean, look at me, aren't I perfectly sober? 'Cause I am Captain Sober
by Bigmamas words April 05, 2011