Meaning of lower mast

low'er mast'

Pronunciation: (lō'ur), [key]
— Naut. Naut.
  1. the lowermost spar of a compound mast, stepped in the hull of a vessel and carrying a topmast and any other upper spars.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.