A sexual act between two consenting homosexuals. One man places his erect penis out and places the palm of his hand on top of the penis. Another man places his erect penis below the other man's penis with the palm of his hand underneath his own penis - both men applying pressure against the two penises. The two men then grind backwards and forwards until both ejaculate.
Hey Mark, my ass is feeling a bit sore tonight, fancy a knobjob instead?
A derogatory job for someone who is much displeasing to the people around him, unlike a normal "knobjob" which is very pleasing, even to crowds of people, which is then referred to as "bukkake".
I can't believe Jerry punched that toddler, what a real knobjob. Great left hook though.