What does "mug it up" mean? Make a nice face? Make a face with an exaggerated expression?
Something else altogether?

Question

What does "mug it up" mean? Make a nice face? Make a face with an exaggerated expression? 
Something else altogether?

Answer

I have hear  "mug it up" to mean to get a lot to eat. Also  it is used as "mug up" meaning the same thing.
"I'm going to mug  it up when I get home."
"I can't wait  to mug up at McDonalds."

Answer

Here, in this case, I think your second answer is right. The Free Dictionary says:

to mug it up (US) 
(= grimace) → gesticular,

and grimace is to make a face like you are in pain or disgusted

Mug here, in Britain, is slang for a foolish person, e.g.: "John, you were a real mug lending him £500 - he won't give it back!"

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