E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Splice the main-brace, in sea language, means to take a draught of strong drink to keep the spirits up, and give strength for extra exertion. The main-brace is the rope by which the mainyard of a ship is set in position, and to splice it, in a literal sense, when the rope is broken or injured, is to join the two ends together again.