Meaning

noun in the Roman calendar: the 15th of March or May or July or October or the 13th of any other month

originFrom Middle English ides, idus, from Anglo-Norman and Old French ides, from Latin īdūs, a fourth-declension plurale tantum, from the Latin practice of treating most recurring calendrical days as plurals. The Latin term is cognate with Oscan eiduis, both perhaps deriving from an unknown Etruscan term. Middle English and Old French also used the singular form ide.

Etymology from Wiktionary, CC BY-SA 3.0