The only free and comprehensive online etymological dictionary of the Spanish language
en (Preposition) "in," "at," "about," "against," "within," "into"

10th cent. From Latin in 'id.' (Archaic Latin en). From Proto-Italic *en 'id.' From Proto-Indo-European *h1en 'id.'

Romance: Western Vulgar Latin: Portuguese em, Catalan en, French en, Italian in; Eastern Vulgar Latin: Aromanian ãn, Romanian în

Italic: Oscan en, Paelignian i

Indo-European: Celtic: Old Irish i, Old Welsh in, Old Breton en, Cornish yn; Germanic: Gothic in, Old Norse í, Old Saxon in, Old High German in, Old English in (English in); Balto-Slavic: Old Church Slavonic vъn, Russian vo, Lithuanian į, Old Prussian en; Hellenic: Ancient Greek ἐν (en); Armenian: i; Tocharian: A yn-, B in-
enamorado (Adjective, Noun) "enamored;" "lover"

15th cent. From enamorar.
enamorar (Verb) "to be in love"

15th cent. From en- "in" and amor.
encantado (Adjective) "delighted;" "charmed"

14th cent. From encantar.
encantar (Verb) "to delight," "to enchant;" (less literally) "to love"

14th cent. From Latin incantare "to chant (as a mystical ritual)," "to enchant (with a spell);" "to sing." Composed of in- (see en-) and cantare "to sing" but also "to chant," "to recite" (see cantar).

Romance: Western Vulgar Latin: Portuguese encantar, Galician encantar, Catalan encantar, French enchanter, Italian incantare; Eastern Vulgar Latin: Aromanian ncãntu, Romanian încânta
encima (Preposition, Adverb) (of location) "on top of;" (as a conjuctive) "furthermore"

13th cent. From Latin in- "on" (see en-) and cyma "peak" (see cima).
encontrado (Adjective) "opposed;" "found"

Very early 13th cent. The sense of "opposed" is original, as the meaning of econtrar was to encounter in combat. From encontrar.
encontrar (Verb) "to find"

Very early 13th cent. According to Roberts (2014), the Old Spanish sense of the word meant to find oneself in the midst of conflict. Compare Old French encontrer "to meet in battle." From Late Latin incontrare 'id.' From in- (see en-) and contra "against" (see contra).
enemigo m. (Noun, Adverb) "enemy," "hostile"

12th cent. From Latin inimicus 'id.' From in- "not" and amicus "friend" (see in- and amigo respectively).

Romance: Western Vulgar Latin: Asturian enemigu, Portuguese inimigo, Catalan enemic, French ennemi, Italian nemico; Eastern Vulgar Latin: Romanian inamic; Sardinian: anemigu

Indo-European: Celtic: Old Irish námae "enemy" (n̥-h2mh3- "one not taken hold of," "non-friend")
energía f. (Noun) "energy"

Early 17th cent. Borrowed from Late Latin energia 'id.,' itself borrowed from Ancient Greek ἐνέργεια ‎(enérgeia) "activity." Derived from ἔργον (ergon) "work," "labor." Mycenaean we-ka-ta. From Proto-Indo-European *u̯erǵ- "to work."

Hellenic: Doric ϝέργον (wérgon)

Indo-European: Celtic: Gaulish, first element in uergo-bretus "chief magistrate;" Germanic: Old Norse verk, Old High German werc, Old Saxon werk, Old English weorc (English work); Armenian: gorc; Indo-Iranian: Sanskrit ū́rj- "strength," Avestan varəzəm