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1.
The oudacious and drunken act of inserting ones hand down the back of a womans jeans, in an attempt to reach around and grab the crotch. Only applicable to woman who are strangers.
So my buddy just reached down the back of that 40 year olds pants when she was leaning forward at the bar and gave her the Captains Handshake!
by Jeff Steves June 25, 2011
 
2.
A quite objectionable move usually performed by an intoxicated male on an unwitting female in which he slides his hand down the back of her pants, continues moving his hand past her ass and through her legs eventually reaching around frontside to grab her vagina.

The phrase is derived from the only person who could possibly get away with such an offensive action, the Captain.
Man #1: "Why did that chick slap John Paul across the face?"
Man #2: "He tried to give her the Captain's handshake"
Man #1: "Makes sense"
by The Gov'nor June 25, 2011