When a person is so annoying that they get you really mad and the only way to stop them doing what they are doing is to give them a single powerful punch to the face which knocks them unconscious, they are now said to be "knocked the fuck out". This is only achieved if it is a single punch which knocks them unconscious.
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.