Green’s Dictionary of Slang

poison oneself v.

to take a drink.

1885		Bulletin (Sydney) 22 Aug. 14/3: He was journeying in a coach [...] and had for a fellow-passenger a genial-souled gentleman who got off at every pub. to sluice his neck with the liquid damnation, and each time tendered the Rev. Julius a pressing invite to come and poison himself, which the latter coldly declined.
1893		H. Lawson ‘The Darling River’ in Roderick (1972) 90: He invited the boys to ‘pisen themselves’.