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Knuck = Knuckles.
Buck = A male deer.

Knuck if you buck = Used when hunting. Simply hit your knuckles together if you see a male deer to create the effect of 2 deer fighting. (The antler effect.)
*deer approaches*

"Quick, knuck if you buck!"

...the buck (male deer) comes, and you can do as you wish with it.
by T-pain November 16, 2005
A secert term amongst homosexuals. (A question inquiring if you scream while having butt sex preformed upon you.)
Timmy: "Knuck if you Buck?"
Tommy: "Hell yeah!"
by TruGangsta October 07, 2004