Lewis & Short

nī (old orthography nei, v. in the foll.), adv. and conj. [identical with ne and the prim. form, whence nisi, i. e. si ni].

  1. I. Adv., like ne, an absolutely negative particle, not.
    So only in the combinations,
    1. A. Quid ni? or, in one word, quidni? why not? quid ego ni ita censeam? Plaut. Mil. 4, 3, 27: quid ego ni fleam? id. ib. 4, 8, 1: quidni, inquit, meminerim? etc., Auct. ap. Cic. de Or. 2, 67, 273 et saep.; v. quidni.
    2. B. Nimirum, lit. not wonderful; v. nimirum.
  2. II. Conj., like ne, in imperative and intentional clauses, not, that not (ante-class. and poet.): ni quid tibi hinc in spem referas, Plaut. Ep. 3, 2, 3: vinum aliudve quid ni laudato, Varr. ap. Non. 281, 31: Numa constituit, ut pisces, qui squamosi non essent, ni pollucerent … ni qui ad polluctum emerent, Cass. Hem. ap. Plin. 32, 2, 10, § 20; cf.: QVEM QVISQVE EORVM AGRVM POSIDEBIT, INVITIS EIS NIQVIS SICET NIVE PASCAT NIVE FRVATVR, etc., Inscr. Orell. 3121, p. 40: IS EVM AGRVM NEI HABETO NIVE FRVIMINO, ib.: ROGO PER DEOS, NI VELITIS OSSA MEA VIOLARE, Inscr. Grut. 996, 12; Cenotaph. Pis. ap. Inscr. Orell. 643: (lege) edictā flemus diu, ni nos divideret, Prop. 2, 7, 2: obstabat vallum, ni instent acies, Sil. 1, 374 (al. ne): monent … ni teneant cursus, Verg. A. 3, 686 (antiqui ni pro ne ponebant, Serv. ad loc.).
  3. III. As a conditional negative (= si non, nisi), if not, unless, but that.
    1. A. In gen.
          1. (α) With indic.: SI IN IVS VOCAT, NI IT, ANTESTATOR, IGITVR EM CAPITO, if he summon him before court, if he go not, then he shall, etc., Tab. XII. 1 init.: nam ni illum recipit, nihil est quo me recipiam, Plaut. Capt. 1, 1, 35: ni offerumentas habebis plures in tergo tuo, etc., id. Rud. 3, 4, 48: mirum ni domi est, Ter. And. 3, 4, 19: moriar ni puto, etc., Cic. Fam. 7, 13, 1: ni frustra augurium vani docuere parentes, Verg. A. 1, 392: si is homo moritur, probe factum … ni moritur, tum, etc., Liv. 8, 10, 12; 13.
          2. (β) With subj.: quid ploras, pater?
            Mirum ni cantem: condemnatus sum, it’s a wonder I don’t sing (ironically), Nov. ap. Cic. de Or. 2, 69, 279: dicerem, quae ante futura dixissem, ni vererer, ne, etc., Cic. Fam. 6, 6, 4: ni ita se res haberet, id. Tusc. 5, 39, 115; cf. id. Fin. 3, 20, 66: ni tamen exciderit, id. poët. Fragm. ap. Quint. 8, 6, 73: ni faciat, Verg. A. 1, 58: omina ni repetant Argis numenque reducant, id. ib. 2, 178: nec Boi detrectāssent pugnam, ni fama … animos fregisset, Liv. 32, 31, 2: ea se dicturum, quae ni fiant, nulla sit pacis condicio, id. 32, 33, 2; 2, 22, 1; 1, 22, 6.
    2. B. In partic.
      1. 1. In formal lang. of law, in agreements, promises, stipulations, etc.: cum is sponsionem fecisset ni vir bonus esset, Cic. Off. 3, 19, 77: judicem ferre, ni vindicias dederit, Liv. 3, 57: tum illud quod dicitur sive nive irrident, etc., Cic. Caecin. 23, 65: cedo qui cum habeam judicem, Ni dolo malo instipulatus sis. nive etiam dum siem Quinque et viginti annos natus, Plaut. Rud. 5, 3 (4), 25 (v. the passage in connection); so id. ib. 3, 4, 9: id ni fit, mecum pignus, si quis volt, dato in urnam mulsi, that it is not so, bet me, etc., id. Cas. prol. 75; id. Ep. 5, 2, 35; id. Poen. 5, 4, 72.
      2. 2. Ni quis scivit (if any one has not voted), the name of a centuria created by Servius Tullius for those to vote in who had not voted in their own centuriae, acc. to Paul. ex Fest. p. 177 Müll.
      3. * 3. For nisi quod, Amm. 22, 11 fin.
      4. 4. Ni forte, for nisi forte: ni forte satius est, etc., Curt. 5, 25, 12 (Foss, nisi), Quint. 11, 2, 27.