Conversation:

Ok - I may be dumb - but what exactly is a duff? Sounds like a bottom from context, but maybe it's an East European chair?

No, you're right. A duff is a person's bottom. "Get off your duff" is a phrase that means, "Stop fooling around. Get up and get moving."

Otherwise, I'd have to say that a "duff" is a piece of lint lodged in one's ear canal.

Let's not forget it is also a brand of beer on The Simpsons.

*sings*

Duff beer for me,
Duff beer for you!
I'll have a duff,
You have one, too!

Tee hee hee...