The only free and comprehensive online etymological dictionary of the Spanish language
nervio m. (Noun) "nerve"

13th cent. Borrowed from Medieval Latin nervium 'id.,' from Latin nervus of the same meaning. From Proto-Italic *nēuro- "nerve," "sinew." From Proto-Indo-European *snéh1- 'id.'

Romance: Western Vulgar Latin: Asturian nerviu, Portuguese nervo, Galician nervio, Catalan nervi, French nerf, Italian nervo

Indo-European: Hellenic: Ancient Greek νεῦρον (neûron) "sinew;" Armenian: neard "sinew;" Indo-Iranian: Sanskrit snā́van "sinew," Young Avestan snāuuarə "sinews;" Tocharian: B ṣñaura "sinews"
nervioso (Adjective) "nervous"

Very late 15th cent. From nervio.

Romance: Western Vulgar Latin: Asturian nerviu, Portuguese nervo, Galician nervio, Catalan nervi, French nerf, Italian nervo
ni (Adverb) "neither;" "not even," "even"

From Latin nec "not," an apocopated form of neque "and not," from ne "not" and -que "and." See no and -que respectively for further etymology.

Romance: Western Vulgar Latin: Asturian nin, Portuguese nem, Galician nin, Catalan ni, French ni, Italian
nieve f. (Noun) "snow"

13th cent. From Vulgar Latin *neve 'id.' From Latin nivem, accusative of nix 'id.' From Proto-Italic *snik- 'id.' A feminine from Proto-Indo-European *sni̯gwh- 'id.'

Romance: Western Vulgar Latin: Asturian nieve, Portuguese neve, Galician neve, Catalan neu, French , Italian neve ; Eastern Vulgar Latin: Aromanian neao, Romanian nea ; Sardinian: ni

Indo-European: Celtic: Old Irish snigid "to snow," Middle Welsh nyf "snow;" Germanic: Gothic snaiws "snow," Old Norse snær 'id.,' Old High German snēo 'id.,' Old Saxon snēo 'id.,' Old English snāw 'id.' (English snow); Balto-Slavic: Old Church Slavonic sněgь "snow," Russian sneg 'id.,' Czech sníh 'id.,' Polish śnieg 'id.,' Slovene snẹ̑g 'id.,' Old Prussian snaygis 'id.,' Lithuanian sniñga "it snows," Latvian snìegs "snow;" Hellenic: Ancient Greek νίφειν (níphein) "to snow;" Indo-Iranian: Sanskrit asnihat "to lie down," Young Avestan snaēža- "to snow"

The snow-lexicon of the Indo-Europeans was basic and broad; of few words and with little nuance. As Mallory & Adams (2014) point out, however, the language of Proto-Uralic - the Indo-European's neighbors to the north - was varied and complex.
ninguno (Adjective) "none"

10th cent. Old Spanish niguno, nenguno. From Latin nec unum "not one." For the etymology of nec, see ni; for the etymology of unum, see uno.

Ningun is an apocopation of ninguno, used before masculine singular nouns.

Romance: Western Vulgar Latin: Portuguese nenhum, Catalan ningú
niño (Noun) "child"

12th cent. Old Spanish nino, also in the name Ninno Macellero; 13th cent. niño. From Vulgar Latin *ninnus 'id.' Of unknown origin. Possibly borrowed from a pre-Indo-European language or of nursery origin. Consider also a late Latin word nonnus "monk" but also "elderly person" (see ñoño).

Also the origin of the surname Niño, first given as a nickname and later formalized as a surname.

Dialect Variants: Miranda de Duero nino, Colunga nin (ninus)

Romance: Western Vulgar Latin: Italian nino
nivel m. (Noun) "level"

15th cent. Borrowed from Catalan nivell 'id.,' dissimilated from Vulgar Latin *libellum 'id.,' from Latin libra "balance" (see libra).

Romance: Western Vulgar Latin:Portuguese nivel, Galician nivel, Catalan nivell, French niveau, Italian livello ; Eastern Vulgar Latin: Romanian nivel
-no Adjective-forming suffix.

From Latin -nus 'id.' From Proto-Italic *-no- 'id.' From late Proto-Indo-European *-n-ó- 'id.'

Indo-European: Germanic *-ana-: Gothic -an, Old English -en, Old High German -an, -en, Old Saxon -anBalto-Slavic *-nas: Old Church Slavonic -nъ, -nije, Russian -nyj, -n'e, Czech -ný, -ní, Slovak -ný, Polish -ny, Slovene -n, Bulgarian -n; Hellenic: Ancient Greek -νος ‎(nos)
no (Adverb) "no," "not"

10th cent. Old Spanish non. From Latin non 'id.," from Old Latin noenum "not one," from ne "not" and oinos "one" (see uno).

Romance: Western Vulgar Latin: Asturian nun, Portuguese não, Galician non, Catalan no, French non, Italian no ; Eastern Vulgar Latin: Aromanian nu, Romanian nu ; Sardinian: no
nocente (Adjective) "harmful;" "guilty"

From Latin nocentem, accusative of nocens 'id.' From nocere "to harm." From Proto-Italic *nok-eje- "to cause death," especially a murderous death. From Proto-Indo-European *noḱ-ei̯e- "to cause death," but originally a euphemism that meant "to cause to disappear." Derived from the root *neḱ- "death," "disappearance."

Romance: Western Vulgar Latin: Galician nocer, Catalan noure, French nuire, Italian nuocere

Indo-European: Indo-Iranian: Sanskrit nāśáya- "to make disappear," Old Persian vināθayatiy "he damages"