Definition
To have an alcoholic drink.
Origins
From splice + mainbrace. Splicing the mainbrace, a heavy piece of running rigging that supported the main yard, was a skilful and arduous task, so that the men responsible would be rewarded with an extra tot of rum. Nowadays, a ceremonial order to pour a drink for a ship's company.
How People Actually Use It
In the English Navy grog was 1/4 rum and 3/4 water. When the sailors would bring in the main sails, it was such a tough job that they would be rewarded with a 'double' which was 1/2 rum and 1/2 water. This is now a way to say 'have a double.'
"You did a great job, splice the main brace."
Source: Urban Dictionary