Green’s Dictionary of Slang

1. an examination, a test.

1853		Maxwell in L. Campbell Life (1882) 191: I was down after the Mug* with Tayler’s uncle in Suffolk. *Trinity College Examination .
1937–84		Partridge DSUE (1984) 764: from ca. 1852.
1986		Eble Campus Sl. Mar. 7: mug – a thing: I made an ‘A’ on that mug (mug=test).

2. preparation (e.g. for an examination).

1901

T.B. Reed Master of Shell 11: I’m doing some of the mug, so as not to be all behind .

3. a schoolchild or student who works hard.

1888		Berkshire Gloss. 114: Mug, as a schoolboy’s expression to work hard, and one who does so is somewhat contemptuously termed ‘a mug’ by others .
1901		Daily News 6 Feb. 9/5: At the University of Oxford a ‘mug’ is a person who is not given to sport, or any indulgence, but who reads a great many books which he doesn’t understand .
1937–84		Partridge DSUE (1984) 764: from ca. 1880.