Meaning

noun the quantity that can be held in a mug

noun a person who is gullible and easy to take advantage of

noun the human face (`kisser' and `smiler' and `mug' are informal terms for `face' and `phiz' is British)

originEarly 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…

Etymology from Wiktionary, CC BY-SA 3.0