Necessary
late Middle English: from Latin necessarius, from necesse ‘be needful’.
wiktionary
From Middle English necessarye, from Old French necessaire, from Latin necessārius(“ unavoidable, inevitable, required”), variant of necesse(“ unavoidable, inevitable”), probably from ne or non cessum, from the perfect passive participle of cēdō(“ yield; avoid, withdraw”); see cede.
Older use as a noun in reference to an outhouse or lavatory under the influence of English and Latin necessārium, a medieval term for the place for monks' "unavoidable" business, usually located behind or attached to monastic dormitories.
etymonline
necessary (adj.)
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").
necessary (n.)
also necessaries, mid-14c., "that which is indispensable; needed, required, or useful things; the necessities of life; actions determined by right or law; that which cannot be disregarded or omitted," 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.).
