Wiktionary
splice the mainbrace
vb. (context nautical idiomatic English) To have a drink.
Wikipedia
Splice the mainbrace
"Splice the mainbrace" is an order given aboard naval vessels to issue the crew with an alcoholic drink. Originally an order for one of the most difficult emergency repair jobs aboard a sailing ship, it became a euphemism for authorized celebratory drinking afterward, and then the name of an order to grant the crew an extra ration of rum or grog.