Green’s Dictionary of Slang

aboard adv.

in one’s stomach, esp. of drink.

1834		N. Ames ‘Old Cuff’ Old Sailor’s Yarns 68: Old Cuff, who was ashore on liberty [...] had his ‘beer aboard’.
1851		A.T. Jackson Forty-Niner (1920) 47: Some of the men got too much gin aboard and a quarrel started.
1908		H. Lawson ‘Roll Up at Talbragar’ in Roderick (1972) 746: A new-chum suggested that Jack had more than he thought aboard and was thrown from his horse.
1936		W.J. Blackledge Legion of Marching Madmen 23: [A]rrack [...] has a devil of a kick. Tiger, being a young fool who would try anything once, took more aboard than was good for him.

In phrases

get aboard (v.)

to be drunk, to get drunk.

1801

C. Dibdin Yngr Song Smith 71: He wou’d now and then get so aboard of the grog!