Mug is a slang term for face in English. According to most dictionaries, it comes from those old mugs which were decorated with faces like Toby Jugs.
Daniel Cassidy, author of an atrocious piece of pseudo-scholarship called How The Irish Invented Slang, disagrees. He believes that it comes from the word muc, meaning pig. It is worth quoting his claim in full, as it clearly shows Cassidy’s poor scholarship and dishonesty.
“Muc, n., a pig; anything resembling a pig or hog; (of person) a piggish, hoggish individual, a swine; a scowl; a beetling brow; a scowling face; a piggish face. Múchna; n. a surly appearance; piggish scowl. Muc ar mala, a scowl, a beetling of brows, a piggish mug.
Most Anglo-American dictionaries derive the slang word mug from an English drinking mug with an ugly face painted on it. In Irish American vernacular, a mug (muc, a scowling, beetle-browed face) is a pig-faced mucker.”
The first point to make is that múchna is nothing to do with muc. Múchna comes from múch, meaning to extinguish or suppress. And it doesn’t sound anything like mug, so it is completely irrelevant here.
Then there is the problem of what muc means. If it meant ‘a scowling face, a piggish’ face, then it would be a pretty good candidate for the origin of mug. So does it?
If you’ve read the other posts in this blog, you’ll know what to expect. Cassidy was a pathological liar and muc does not mean a face … scowling, beetle-browed, smiley or any other kind. Muc means a pig, or a bulge which is rounded or pig-shaped. Muca sneachta are snowdrifts. When someone frowns, they get a small rounded bulge on their forehead, which in Irish is called muc ar gach mala (a bulge on each brow). It is used in this way but the phrases quoted above “a scowling face; a piggish face … a piggish mug … a scowling, beetle-browed face” are not true definitions of muc. They are Cassidy inventions. If you asked somebody in Irish why they had a muc on them (without the ar gach mala bit), they would look at you in puzzlement and say that they don’t have a pig on them. What Cassidy is saying is a little like saying that ‘laughter’ can be used in English to mean a wrinkled face because people talk about laughter lines. It is pure and total nonsense.