necessity n.

late 14c., necessarie, "needed, required; essential, indispensable; such as must be, that cannot be otherwise; not voluntary or governed by chance or free will," from Old French necessaire "necessary, urgent, compelling" (13c.), and directly from Latin necessarius "unavoidable, indispensable, necessary," from necesse "unavoidable, indispensable," originally "no backing away," from ne- "not" (from PIE root *ne-) + cedere "to withdraw, go away, yield" (from PIE root *ked- "to go, yield").

The etymological sense is of that from which there is no evasion, that which is inevitable. Necessary house "privy" is from 1610s (compare Medieval Latin necessarium "a privy"). Necessary evil is from 1540s (the original reference was to "woman").

"one who maintains the doctrines of philosophical necessitarianism," which holds that the action of the will is subject to the law of cause and effect (opposed to philosophical libertarianism), 1754, from necessity + -arian. As an adjective from 1739. Related: Necessitarianism.

1620s, "force irresistably, compel, oblige," also "make necessary, render unavoidable," from Medieval Latin necessitatus, past participle of necessitare "to render necessary," from Latin necessitas "compulsion; destiny" (see necessity). Earlier verb in English was necessen (late 14c.). Related: Necessitated; necessitates; necessitating.