One day, some stoned happy chappy was thinking of things that rhymed with dandruff, and by some happy coincidence, along came a man inflicted with the 'druff, to which stoned happy chappy cried, 'Mandruff!'
And so it was. Do you see?

Dandruff inconveniently located on the head of an older man.

Dried semen in hair that looks just like dandruff. But isn't.

dead skin, concealed beneath facial hair, that flakes away when you play with it, leaving a trail of snow down the front of your shirt. very similar to the concept of dandruff, but mostly only applicable to men.