Green’s Dictionary of Slang

mug (up) v.1

[SE mug / mug n.1 (1d)]

(US) to eat or drink heartily, to have a good meal.

[US]J. Brougham Basket of Chips 369: The private soakers as locks themselves up with the grog, and mugs themselves, alone.
[UK]Kipling Captains Courageous 161: No reg’lar meals fer no one then. ‘Mug-up when ye’re hungry, an’ sleep when ye can’t keep awake.’.
[US]Scribner’s Mag. XXIX 498/2: Let Martin and me mug up and get over near the fire to dry out [DA].
[US]Ersine Und. and Prison Sl.