A fist bump type action used by two friends when confiding in one another.
When you "put it on knucks" this fist bump lets both you and your friend know that what is being said is the truth, and that its not to be spoken of to anyone else.
Kieran: Guess what I just did!
Ksystyk: What did you just do?
Kieran: I just hooked up with that cute new girl from math class
Ksystyk: Put it on Knucks
Kieran: boooh yaah!
Ksystyk: You're so lucky dude
"Put it on Knucks" is a type of fist bump done by two friends. When this fist bump occurs, or is brought up to be done it lets the two friends know they're telling the truth, being serious, and that they're confiding in one another.
You: Dude, I just hooked up with Ms. White
Me: You're such a liar.. yea right.
You: I'm being honest but you can't tell anyone I told you.
Me: Put it on Knucks
*Fist bumps each other*
Me: Omg, you're so lucky.. did you get pics?