Meaning

noun a gathering of military personnel for duty

noun compulsory military service

verb gather or bring together

originFrom Middle English musteren, borrowed from Anglo-Norman mostrer, Middle French monstrer, moustrer (whence the noun monstre, which gave the English noun), from Latin mōnstrō (“to show”), from moneō (“to admonish”). Cognate with French montrer (“to show”), Italian mostrare (“to show”), Spanish mostrar (“to show”). See also monster.

Etymology from Wiktionary, CC BY-SA 3.0