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mug /mahg/ n [C] 1 a large cup with straight sides used for drinking tea, coffee etc. 2 a large glass with straight sides and a handle, used especially for drinking beer: rugby players swilling mugs of beer at the bar 3 also mugful a mug and the liquid inside it: Two mugs of tea, please. 4 BrE spoken someone who is stupid and easy to deceive: I expect they'll find some poor mug to buy their car. 5 spoken a face: What an ugly mug! 6 a mug's game BrE spoken something that is not likely to be successful or profitable: Gambling is a mug's game, Jonah.

mug v mugged, mugging 1 [T] to attack someone and rob them in a public place: A lot of people won't go out alone at night for fear of being mugged. 2 [I] AmE informal to make silly expressions with your face or behave in a silly way, especially in a photograph or a play: Scotty's always mugging for the camera.
mug up phr v [I, T] BrE informal to study something very hard, especially when preparing for an exam: mug sth up He's got to mug up some facts about pollution [+ on] She's mugging up on Racine for her French paper.

mug.ger /'mahgger/ n [C] someone who attacks people and robs them in a public place

mug.ging /'mahgging/ n [C, U] an attack on someone in which they are robbed in a public place: There has been an increase in muggings at the park.

mug.gins /'mahgginz/ n [singular] spoken BrE used jokingly to mean yourself, when you know you have been stupid and let other people treat you unfairly: Everyone disappeared after supper, leaving muggins here to do the washing-up.

mug.gy /'mahggie/ adj muggier, muggiest informal muggy weather is unpleasantly warm and the air seems wet: The air was muggy and damp, threatening a storm later. --mugginess n [U]

Definitions courtesy of Longman Dictionary of Contemporary English.

And here's the definition from an alternative dictionary.

mug v mugged, mugging 1 [T] to study very hard: Students love to mug in the school library. 2 [T] SgpE student colloquial to cram all information from notes and textbooks into one's brains in the hope that they turn out relevant during exams

mug.ger /'mahgger/ n [C] someone who is very diligent and mugs a lot, often staying up entire nights to study

closet mugger /'.. ,../ n [C] someone who mugs a lot but does so in the privacy of his own room so that other people will not find out the extent of his mugging

mug.ging /'mahgging/ n [C, U] a group activity in which muggers gather at a public place, usually a library, to study together: There has been an increase in mugging sessions ever since exam season started.

Mugging sessions are classified into two types: productive and non-productive. They usually start out non-productive, but evolve to become productive as the exam season nears. Non-productive mugging sessions are usually accompanied with relatively high noise levels and a lot of laughter, while hair-clutching and frantic scribbling or highlighting are observed during productive sessions. Productivity has a positive correlation with stress levels, as recent studies have shown, but causative effects have yet to be proven.