vt.: To penetrate coarsely during sex, esp. with mostly the head of the penis, rather than inserting the entire shaft
"Knob" as a transitive verb: 1) Insecure about his short shaft but proud of his wide cockhead, he knobs his lover's pussy in order to accentuate the positive. 2) Mr. Doe couldn't stop fantasizing while making love with his wife that for once he'd like to knob her hole like the stud he really is.
2. Also can be used as a term similar to goof, tool or moron.
2.Dude, did you hear Bill locked the keys in his car again - what a knob!