Etymology: Early 16th century (originally Scots and northern English, denoting "earthenware, pot, jug"), of unknown origin, perhaps from North Germanic (compare Swedish mugg (“mug, jug”), Norwegian mugge (“pitcher, open can for warm drinks”), Danish mugge), or Low German mokke, mukke (“mug”), German Low German Muck (“drinking cup”), Dutch mok (“mug”), also of unknown origin. Perhaps related to Old English muga (“stack”) and Old Norse múgr (“mass, heap (of corn)”). Compare also Middle English mug, mog (“a measure of salt”).

Easily fooled, gullible.
“"Great heavens! Is it?" Drummond helped himself to marmalade. "And to think that I once pictured myself skewering Huns with it. Do you think anybody would be mug enough to buy it, James?"”
Uninteresting or unpleasant.
“But anyways, I stayed back a second year and my papa was visiting when momma was opening de report card. Papa gave me de muggest moment in my life like.”
“So... complaining about Bermuda being boring/mug on a constant does what exactly?”

noun

Etymology: Informal variant of motherfucker.

Motherfucker (usually in similes, e.g. "like a mug" or "as a mug")

verb

To strike in the face.
“Madgbury showed game, drove Abbot in a corner, but got well Mugg'd.”
“1857, "The Leary Man", in Anglicus Ducange, The Vulgar Tongue And if you come to fibbery, You must Mug one or two,”
To assault for the purpose of robbery.
To exaggerate a facial expression for communicative emphasis; to make a face, to pose, as for photographs or in a performance, in an exaggerated or affected manner.
“The children weren't interested in sitting still for a serious photo; they mugged for the camera.”
“Her dress is too obviously "costume," and Ford-Smith mugs too broadly for us to forget, as we should, that Katie is a theatrical invention.”
To photograph for identification; to take a mug shot of.
“Perhaps if I told you that there were forty arrests made in one day here, you will realize that with this small equipment available in the Sheriff's Office, it is very difficult to get mugs. These people are being mugged as they are being arrested but with an entirely inadequate force at work prints have not as yet been made.”
“The Bat—they called him the Bat.[…]. He'd never been in stir, the bulls had never mugged him, he didn't run with a mob, he played a lone hand, and fenced his stuff so that even the fence couldn't swear he knew his face.”
To learn or review a subject as much as possible in a short time; cram.