The only free and comprehensive online etymological dictionary of the Spanish language
dulce m. (Adjective, Noun) "sweet;" "candy"
10th cent. Old Spanish duz, duce. From Latin dulcis "sweet." From Proto-Italic *dulkwi- 'id.' Of unknown origin, presumably borrowed from another language.
Indo-European
Romance
Asturian dulce, Portuguese doce, Galician doce, Catalan dolç, French doux, Italian dolce, Aromanian dultsi, Romanian dulce, Sardinian durke
Hellenic
Ancient Greek γλυκύς (glukys), Mycenaean de-re-u-ko
The adjective "sweet" is original. The noun "candy" is a later innovation, though the innovation may have occurred in very ancient times (the secondary meaning of "dessert" or "candy" spans the Western Romance languages).
dura f. (Noun) "duration"
From Latin durus "hard" (see duro). The meaning broadened in Latin from "hard" to "lasting over a period of time."
Indo-European
Romance
Portuguese duro, Galician duro, Catalan dur, French dur, Italian duro, Romanian dur, Sardinian duru
durante (Adverb) "during"
15th cent. Originally meaning "hard." From Latin durans "enduring" (but originally "hardening;" contrast dura and duro), a past participle of durare "to harden," itself from durus "hard" (see duro).
duro (Adjective) "hard"
13th cent. From Latin durus 'id.' From Proto-Italic *dūro- 'id.' From Proto-Indo-European *du̯h2-ro- 'id.' From the root *du̯eh2- "to endure."
Also the origin of the surname Duro and its diminutive surname Durelo.
Indo-European
Romance
Portuguese duro, Galician duro, Catalan dur, French dur, Italian duro, Romanian dur, Sardinian duru
Hellenic
Ancient Greek δηρός (derós)
Armenian
Armenian erkar
Indo-Iranian
Sanskrit dūrá- "far," Avestan dūrāt̃ 'id.'
dux m. (Noun) "doge"
From Latin dux "general" From Proto-Italic *duk- "lead," From Proto-Indo-European *duk- 'id.,' from the verb *deu̯k-e/o- "to pull." See also dogo (2) and inducir.
Indo-European
Romance
French duc, Italian doge, Romanian duce