Green’s Dictionary of Slang

(Aus.) a drunk.

1959		D. Niland Gold in the Streets (1966) 209: The few drinking at the bar, the old hags and red-nosed blottos at the tables.
2024		J. Meades Empty Wigs (t/s) 789: [S]ome tiddly blottisimo draining every last drop of every last bottle at the end of the party when the party’s over.