c.1300, "to display, reveal, appear," from Old French mostrer "appear, show, reveal," also in a military sense (10c., Modern French montrer), from Latin monstrare "to show," from monstrum "omen, sign" (see monster). Meaning "to collect, assemble" is early 15c.; figurative use (of qualities, etc.) is from 1580s. To muster out "gather to be discharged from military service" is 1834, American English. To muster up in the figurative and transferred sense of "gather, summon, marshal" is from 1620s. Related: Mustered; mustering.