The only free and comprehensive online etymological dictionary of the Spanish language
sueño (2) m. (Noun) "dream"
12th cent. From Latin somnium 'id.' From Proto-Italic *swepnjo- or *swopnjo- "dream." From Proto-Indo-European *su̯épno- or *su̯opno- "sleep" (also the origin of the other meaning of sueño) and *-i̯o-, a putative nominal derivational ending of unclear meaning. See soñar for further etymology.
Indo-European
Romance
Asturian suañu, Portuguese sonho, Galician soño, French songe, Italian sogno, Sardinian sónniu

The -ium in somnium has an unclear history. It is not clear whether is was added during a pre-Latin period (for reasons unknown) or in Proto-Indo-European as an i̯o-derivation, if such a derivation truly existed. The i̯o-derivation may be an i-stem with a thematic vowel (see Meier-Brügger et al., 2003).

Final updates in preparation for a particularly large data update coming at the end of the month.
The Online Etymological Dictionary of Spanish is a free etymological dictionary of the Spanish language launched on August 6, 2016. An etymology is not a definition but a history of a word over time. This site provides an account of the development of the Spanish language as best as historical linguists are able to ascertain in a way that is accessible both by laymen and academics. If you enjoy using this dictionary, please consider donating to help keep this site alive.