" knuckling down (to) "

 [ "knuckled down" ]

1: to physically beat someone down with your fists

  * e.g.,  ... "...there's a good chance ya ass is either gon' get shot, stabbed, or knuckled down, one out of the three." - Prodigy of Mobb Deep (The Infamous Prelude) 

 [ "Knuckle Down" ]

1: To have a firm grip on ones cock and sucking them off with your knuckle's bulging out of your hand to show your hard work.

  * e.g.,  ... Heather popped a vein in her hand after she knuckled down hard on Mikie!!! 

 [ "knuckles down" ]

1: Sly way of saying jerking off.

  * e.g.,  ... I was knuckles down crushin my beef. 

 [ "beef knuckle beat down" ]

1: to have sex

  * e.g.,  ... Remember that girl from biology class? I totally gave her the ol' beef knuckle beat down.