Lewis & Short

nĕcessārĭus, a, um, adj. (comp. necessarior, Tert. Patient. 11; id. Test. Anim. 4 al.) [necesse], unavoidable, inevitable, indispensable, needful, requisite, necessary: necessarium ait esse Opilius Aurelius, in quo non sit cessandum, aut sine quo vivi non possit: aut sine quo non bene vivatur: aut quod non possit prohiberi, quin fiat, Paul. ex Fest. p. 162 Müll.

  1. I. Lit.: necessarius et fatalis, opp. voluntarius, Cic. Phil. 10, 9, 9; cf.: id quod imperatur necessarium; illud, quod permittitur, voluntarium est, id. Inv. 2, 49, 145: necessaria conclusio, id. Top. 16, 60: leges fatales et necessariae, id. Univ. 12: omnia quae sint ad vivendum necessaria, id. Off. 1, 4, 11: senatori necessarium est, nōsse rem publicam, id. Leg. 3, 18, 41.
    So without dat., = necesse est: ne tam necessarium quidem est male meritis quam optime referre quod debeas, id. post Red. ad Quir. 9, 22: castra ponere necessarium visum est, Liv. 21, 58, 6; Plin. Ep. 10, 37, 3; Gai. Inst. 3, 216: necessariā re coactus, by necessity, Caes. B. C. 1, 40: quod tam necessario tempore ab iis non sublevetur, time of need or necessity, id. B. G. 1, 16: cum longius necessario procederent, farther than was necessary, too far, id. ib. 7, 16: res magis necessariae, Cic. Inv. 2, 49, 145: res maxime necessaria, id. Fam. 2, 6, 2: necessarior medela, Tert. Patient. 11: necessarior sententia, id. Test. Anim. 4: necessariores operas, id. Cult. Tem. 1, 5; id. Res. Carn. 31: aliquid necessarius, id. Carn. Christ. 7 med.
    Subst.: nĕcessā-rĭa, ōrum, n., the necessaries of life: Persae armis positis ad necessaria ex proximo vico ferenda discurrunt, Curt. 5, 12, 6: plebes sic adcensa uti … sua necessaria post illius honorem ducerent, Sall. J. 73, 6; Front. Strat. 3, 14, 4.
    1. * B. In partic.: necessariae partes, the private parts, Gai. Inst. 3, § 193.
  2. II. Transf., connected with another by natural or moral ties (of blood, friendship, clientship), belonging, related, connected, bound.
          1. (α) Adj.: cum utrique sis maxime necessarius, Balb. et Opp. ap. Cic. Att. 9, 7, A: victoria hominis necessarii, of a friend, Mat. ap. Cic. Fam. 11, 28, 2; so, homo, of a father-in-law, Nep. Dat. 6: ut a latronibus redimeret necessarias mulieri personas, Dig. 24, 3, 21: necessarius heres = suus heres, the natural heir, who was in the potestas of the deceased (opp. to heres extraneus), Gai. Inst. 2, 37; 3, 153; 156; Dig. 38, 16, 1.
          2. (β) Subst.: nĕ-cessārĭus, i, m., a relation, relative, kinsman, connection, friend, client, patron (cf. necessitudo, II.; syn.: familiaris, intimus): necessarii sunt, ut Gallus Aelius ait, qui aut cognati aut affines sunt, in quos necessaria officia conferuntur praeter ceteros, Paul. ex Fest. p. 162 Müll.; necessarius angustus, a very near relative, Fragm. Jur. Civ. p. 86 Mai.: L. Torquatus meus familiaris ac necessarius, Cic. Sull. 1, 2: in iis necessariis, qui tibi a patre relicti sunt, me tibi esse vel conjunctissimum, id. Fam. 13, 29, 1: nĕcessārĭa, ae, f., a female relative or friend: virgo Vestalis hujus propinqua et necessaria, id. Mur. 35, 73: Cerelliae, necessariae meae, rem commendavi tibi, id. Fam. 13, 72, 1.
            Hence, adv.
      1. 1. nĕcessārĭē (rare), unavoidably, necessarily: necessarie demonstrari, Cic. Inv. 1, 29, 44: comparato cibo, Val. Max. 7, 6, 3.
      2. 2. nĕcessārĭō (the most usual form): necessario reviviscere, Cic. Fam. 6, 10, 5: quibuscum vivo necessario, id. ib. 5, 21, 1: quod necessario rem Caesari enuntiārit, Caes. B. G. 1, 17: copias parat, Sall. J. 21, 1; Quint. 10, 1, 29; 5, 10, 80; Lact. 2, 12.