This is an acronym for Kiss My Balls. Originated when Kiss My Butt/Ass had lost it’s effect. This saying is often used in jest/humor as opposed to in anger or disgust.
"... so I told them they could KMB."
or simply
"You can KMB."
Kick Michael's Bag
If a person shouts KMB, and Michael is in the vicinity, then his bag must be kicked. If Michael is not present, he is to be replaced by another person with a name beginning with M