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 desperationin 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)