old unit of liquid measure equal to one-third of a pipe (42 gallons), 1530s, from Anglo-French ters, Old French tierce (11c.). used in the sense "one-third" in various ways, from Latin tertia, fem. of tertius "a third," from PIE *tri-tyo-, from root *trei- (see three). Also used in Middle English for "a third part" (late 15c.), "the third hour of the canonical day" (ending at 9 a.m.), late 14c., and, in astronomy and geometry, "sixtieth part of a second of an arc."