Green’s Dictionary of Slang

aboard adv.

in one’s stomach, esp. of drink.

[US]N. Ames ‘Old Cuff’ Old Sailor’s Yarns 68: Old Cuff, who was ashore on liberty [...] had his ‘beer aboard’.
[US]A.T. Jackson Forty-Niner (1920) 47: Some of the men got too much gin aboard and a quarrel started.
[Aus]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.

In phrases

get aboard (v.)

to be drunk, to get drunk.

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