This word is a creative understanding for "footless sex". To give a suhanki, one partner picks up the from his or her calf. Then to move your partner back and forth in a psychotic motion. This will create screaming and potentially babies.
Examples from Stepbrothers:

When Dale has sex with his wife while his stepbrother, Derek, is in the other room eating dinner.

Also, this happens later on in the movie when Dale gives Derek's wife a suhanki in the bathroom.
by SP.VP.AP December 2, 2009
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