late 14c., "constraining power of circumstances," from Old French necessité "need, necessity; privation, poverty; distress, torment; obligation, duty" (12c.), from Latin necessitatem (nominative necessitas) "compulsion, need for attention, unavoidableness, destiny," from necesse (see necessary). Meaning "condition of being in need" in English is from late 15c.
To maken vertu of necessite is in Chaucer. Related: Necessities.