splice the main brace

1 (in the Royal Navy) serve out an extra tot of rum.

2 serve out or start to consume alcoholic drinks. British informal

☞ A sailing ship's main brace is a rope attached to its main spar. Splicing it (making a connection in it by interweaving strands) would have been a particularly onerous task, and the phrase probably arose from the custom of awarding sailors who did it an extra ration of rum.