Knuckled on is the act of brown nosing with a boss or someone else of importance. It gets its meaning from the act of gripping someones dick. In extreme cases it is known as "knuckled on with both hands" denoting desperation in the act.
Look at the new guy! He's knuckled on to the foreman!
How did he get away with that screw up? He knuckled on with both hands and blew his way out of it.
"...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)
A form of hand-to-hand combat weaponary, that fits around a persons fingers, to increase the amount of damage created with a punch. These are usually metal.
John: "How come your face is cut?"
Mark: "Adam hit me with knuckledusters on."