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" (from PIE root *ked- "to go, yield"). 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").
mid-14c., "needed, required, or useful things; the necessities of life; actions determined by right or law," perhaps from Old French necessaire (n.) "private parts, genitalia; lavatory," and directly from Latin necessarius (n.), in classical Latin "a relation, relative, kinsman; friend, client, patron;" see necessary (adj.).