The act of thrusting one's knee into another person's butt. The knee doesn't actually go in, but the ensuing pain can be bad, especially if you hit the tail bone. More of a horseplay thing then a sexual thing.
Your friend is walking in front of you. You come up and knee them right in the butt.
Fat Man Syndrome-
The term used to describe the laziness that fat men feel, usually used with sports or house work. Basically, it's when a man uses an excuse such as an injury to avoid work simply because he is feeling lazy.
"Jim was fine at football practice until we had to run windsprints. All of a sudden his FMS kicked in and he told coach he couldn't run because he turned an ankle."
"Bob will come running when I have dinner cooked, but when I ask him to take out the trash he always acts like his old wrist injury is flaring up. I bet it's just FMS."