The only free and comprehensive online etymological dictionary of the Spanish language
definir (Verb) "to define"

Borrowed from Latin definire "to determine." From de- (see de-) and finis "limit" (see fin).

Romance: Western Vulgar Latin: Portuguese definir
definitivamente (Adverb) "definitely"

From definitivo and -mente, an adverb-forming suffix.
definitivo (Adjective) "definitive"

From definir.
deja f. (Noun) "space between two notches"

18th cent. From dejar.
dejado (Adjective) "lazy"

17th cent. One who leaves things undone. From dejar.
dejar (Verb) "to leave"

12th cent. Old Spanish lexar 'id.' Roberts (2014) argues that dejar replaced l- with d- under influence from dar, but this does not explain why d-initial cognates abound in Romance. Far more evincing from Penny (2002) is that lexar represents a learned form from Latin laxare, wheras dejar comes from Vulgar Latin *daxare 'id.' From Latin laxare "to relax" with the l- replaced with d- under influence from Oscan. From the noun laxus (see laja).

Romance: Western Vulgar Latin: Asturian dexar, Portuguese deixar, Galician deixar, Catalan deixar, French laisser, Italian lasciare; Eastern Vulgar Latin: Aromanian lãsari, Romanian lăsare; Sardinian lassare
dejo, deje m. (Noun) "aftertaste;" "peculiar accent"

Very late 15th cent. From dejar.
del (Preposition) "of the"

Contraction of de and el.
delante (Adverb) "in front of"

10th cent. From Old Spanish denante. From de and enante. Enante is from Late Latin inante, from in (see en) and ante "before" (see antes).
demás (Adjective, Pronoun) "remaining," "the rest"

From the Latin phrase de magis 'id.' See de and más respectively.

Used only after a definite article.

Romance: Western Vulgar Latin: Asturian demás, Portuguese demais, Galician demais

Preservation of the Latin phrase de magis in Spanish is remarkable as it is a very archaic feature. De magis is not found in Latin writing after the 2nd cent. BCE (Penny 2002).