Green’s Dictionary of Slang

constitutional n.

[SE constitution, one’s physical and mental state]

1. (Aus.) gin and bitters.

[UK] ‘Drinks and Drinking in Australia’ Town Talk 19 Mar. 547: A constitutional ... Glass of gin and bitters.

2. (US drugs) the first injection of the day.

[US]J.E. Schmidt Narcotics Lingo and Lore.