Learn English – the origin of the word ‘mug up’

First sense

The Oxford English Dictionary's first sense for their 5th verb mug is:

1. intr. To read or study in a concentrated manner. Now freq. with up (on a subject, book, etc.); also formerly with away at, on at.

Their first quotation is from 1848 but I found a slight antedating from in The King's College Magazine of November 1841 (p.235, "Our School Days" by C.H.H.):

If I was industrious, I was sure of a thrashing from some big bully, for " mugging," instead of playing at cricket; if I was idle, I was equally sure of a caning from the master, for not " mugging."

Second sense

Their second sense:

2. trans. To learn (a subject, book, etc.) by hard or concentrated study. Usu. with up.

They date this to 1868 but I found an earlier example from the 1856 Aldershottana: or, Chinks in my hut:

Anon I ordered arms, and let the men stand easy, taking a leaf from Major Coolsneer's book — viz., " Let the men stand at ease ; they are thankful for the indulgence, and look upon it as an act of grace. Meantime, you are quietly mugging up on your next evolution and the proper words of command."

And from 1861's Hills and plains:

The Buffadars always prided themselves upon sticking to the regiment ; they were not like other fellows " mugging up " languages and " sucking " far and wide for staff appointments, doing all they could to shirk their duty.

Here's also an 1866.

Why mug?

The OED says the origin is unknown but perhaps related to an earlier meaning: "To pout, grow sullen; to mope." from the noun mug, a face.

My guess is it comes from having your face, your mug, close to books when studying.