The only free and comprehensive online etymological dictionary of the Spanish language
enfermedad f. (Noun) "sickness"

13th cent. From Latin infirmitatem, accusative of infirmitas 'id.' From infirmus "illness" (see enfermo).

Romance: Western Vulgar Latin: Portuguese enfermidade, Italian infermità
enfermería f. (Noun) "infirmary"

13th cent. Borrowed from Medieval Latin infirmaria 'id.' From Latin infirmus "sick" (see enfermo).
enfermo (Adjective) "sick"

11th cent. From Latin infirmus 'id.,' from in- "not" (see in-) and firmus "strong" (see firme).

Romance: Western Vulgar Latin: Asturian enfermu, Portuguese enfermo, Galician enfermo, French infirme, Italian infermo
enorme (Adjective) "enormous"

15th cent. From Latin enormis "enormous," but originally "irregular." From ex- "outside of" (see ex-) and norma "norm" (see norma).

Romance: Western Vulgar Latin: Portuguese enorme, Galician enorme, Catalan enorme, French énorme, Italian enorme
enseguida (Adverb) "immediately"

From en- and seguida (see seguir).
entender (Verb) "to understand"

From Latin intendere "to focus on." From in- and tendere "to reach for" (see en- and tender respectively).

Romance: Western Vulgar Latin: Asturian entender, Portuguese entender, Catalan entendre, French entendre, Italian intendere; Eastern Vulgar Latin: Aromanian ntindu, Romanian întinde; Sardinian: intèndhere
entero (Adjective) "whole"

13th cent. From Latin integrum, accusative of integer "whole," but literally "untouched." From Proto-Italic *n̥-tagro- "untouched." From Proto-Indo-European *n̥-, a negation prefix (see in- (1)), and *teh2g- "to touch" (see tañer).

Romance: Western Vulgar Latin: Asturian enteru, Portuguese inteiro, Catalan enter, French entier, Italian intero; Eastern Vulgar Latin: Aromanian ntreg, Romanian integru; Sardinian: interu

Italic: Umbrian antakres "untouched"

Indo-European: Celtic: Celtiberian entara, Old Irish eter, Old Welsh ithr, Breton etre, Cornish ynter; Germanic: Gothic undar "between," Old Norse undorn "midday," Old Saxon under "before midday," Old High German untar "between," Old English under "before midday" (English in); Balto-Slavic: Lithuanian į́sčios "womb," Latvian ìekšas "entrails," Old Prussian instran "fat;" Albanian: ndër; Hellenic: Ancient Greek ἔντερα (éntera) "intestines;" Armenian: ənderk' 'id.;' Indo-Iranian: Sanskrit antár, Old Avestan aṇtarə̄̆
entonces (Adverb) "then"

13th cent. From Vulgar Latin *intunces 'id.' Formed from Latin in tunc "in that moment" with -es later added by analogy with other adverbs like quizás. For the etymology of in, see en. Latin tunc is from Proto-Italic *tonk(e) 'id.' From Proto-Indo-European *tom-ḱe 'id.' From *tom "at the moment" and *-ḱe "there."
entrada f. (Noun) "entrance"

12th cent. From Vulgar Latin *intrata 'id.,' originally the passive participle of Latin intrare "to enter" (see entrar).
entrado (Adjective) "aged"

From entrado en años, a metaphorical extension of entering into one's senior years. The past participle of entrar.