there was a young mouse called keith,
who bit a mans knob with his teeth,
it wasnt for leisure,
or sexual pleasure,
but to get to the cheese beneath

Smegma which curdles and forms under the foreskin on a male who has low genital hygiene standards.

The actual name is 'smegma' found behind the foreskin, it usually smells and is fairly stringy and usually yellow in colour

<noun> Offensive Chiefly British Slang

The implication that the subject of the insult is indeed so insignificant as to suggest that they are equal only to the small particles of sweat, semen and urine to be found twix the folds of the foreskin and/or other parts of the penis.

after lack of washing and cleaning, the penis' dirt builds up and sits there happily breeding minging bacteria that stinks like cheese. it is normally crusty and yellow in extreme condtions. you are a knob cheese if your a total twat.

Knob cheese is then dairy product produced on a penis.
Note: this cheese is not from cows

(also bell end brie, helmetdale)
the smelly dirty shit found under the foreskin of tramps and other scum