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1.
The traditional greeting of perverted native Idaho people performed by sticking a finger or thumb up another persons anus instead of shaking hands. This usually comes before even kissing or the common juvenile sexual definitions of first, second, and third base.
"I just met this guy Matt and he gave me an Idaho Handshake on the dance floor!"
by -Brother Dave December 04, 2007
 
2.
The act of putting your finger inside of another persons anus before engaging in any other physical intimate contact (including kissing) with that person.
Did you hear about Matt's new chick? He gave her the Idaho handshake as part of a dare.
by sk355x December 09, 2007