Nautical .

a spar or structure rising above the hull and upper portions of a ship or boat to hold sails, spars, rigging, booms, signals, etc., at some point on the fore-and-aft line, as a foremast or mainmast.

any of a number of individual spars composing such a structure, as a topmast supported on trestletrees at the head of a lower mast.

any of various portions of a single spar that are beside particular sails, as a top-gallant mast and royal mast formed as a single spar.

Also called pillar. the upright support of a jib crane.

any upright pole, as a support for an aerial, a post in certain cranes, etc.

to provide with a mast or masts. ?

before the mast

Nautical . as an unlicensed sailor

He served several years before the mast.