Green’s Dictionary of Slang

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

1855		J. Brougham Basket of Chips 369: The private soakers as locks themselves up with the grog, and mugs themselves, alone.
1897		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.’.
1901		Scribner’s Mag. XXIX 498/2: Let Martin and me mug up and get over near the fire to dry out [DA].
1933		Ersine Und. and Prison Sl.