late 14c. "needed, required, essential, indispensable," 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" + cedere "to withdraw, go away, yield" (see cede). The root sense is of that from which there is no evasion, that which is inevitable. Necessary house "privy" is from c.1600. Necessary evil is from 1540s (the original reference was to "woman").