Yardarm is a nautical term referring to the outer extremities of a yard on a sailing ship. Here’s how it fits into the structure of a ship: Definition: A yard is a horizontal spar from which sails are set, typically mounted on the mast. The yardarm is essentially the outer end of this spar. Usage: Historically, the term "yardarm" is perhaps best known from the expression "sun over the yardarm," used to denote an appropriate time of day (traditionally when the sun reaches the yardarm) to begin drinking.