Brewer's: Before the Mast
To serve before the mast. To be one of the common sailors, whose quarters are in the forward part of the ship. The half-deck is the sanctum of the second mate, and, in Greenland fishers, of the spikeoneer, harpooners, carpenters, coopers, boatswains, and all secondary officers; of low birth.
“I myself come from before the mast.” —Sir W. Scott: The Antiquary, chap. xx.