Crossword Genius

Rob him, the fathead! (3)

Ross

I believe the answer is:

mug

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'rob' is the definition.
(slang)

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for mug that I've seen before include "Tankard" , "Slangy victim, by gum" , "A drinking vesel or a slangy victim" , "Vessel for a bit of a fool" , "Cup; stupid person" .)

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