The only free and comprehensive online etymological dictionary of the Spanish language
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