The only free and comprehensive online etymological dictionary of the Spanish language
acuerdo m. (Noun) "agreement"

13th cent. A noun derived from the past participle of acordar.

Romance: Western Vulgar Latin: Asturian acordes, Portuguese acordo, Catalan acord, French accord, Italian accordo
a(d)- Prefix "at," "towards"

From Latin ad-. -d- is eliminated under the influence of some consonants. For example, abogado from Latin advocatus. From Proto-Italic *ad- 'id.' Grammaticalized prefix from the preposition *ad (see a).

Romance: Western Vulgar Latin: Asturian a-, Portuguese a-, Galician a-, Catalan a-, French a-, Italian a-; Eastern Vulgar Latin: Romanian a-

Italic: South Picene ad-, Oscan ad-, ar-, Volscian ar-, Umbrian ař-, -ař

Indo-European: Celtic: Gaulish ad-, Old Irish ad-, Welsh ad-; Phyrgian: αδ- (ad); Germanic: Old High German az-
adelante (Adverb, Interjection) "forward;" "enter!"

10th cent. From a(d)- and delante.
además (Adverb) "furthermore"

13th cent. From a(d)- and demás.
adentro (Adverb) "inside"

12th cent. From a and dentro.

Romance: Western Vulgar Latin: Portuguese adentro
adiós (Interjection) "goodbye"

15th cent. From an earlier phrase in Old Spanish á Dios seas.

The phrase derives from the act in Romance languages of bidding one "to God" at their departure.

Romance: Western Vulgar Latin: Asturian adiós, Portuguese adeus, French adieu, Catalan adéu, Italian addio

Archaic madios! meaning "by God!" is likely a contraction of a "God help me" phrase. Compare French maidieu and m'aide dieu, Old French si m'ait dieus.
-ado Suffix Indicating likeness or relatedness.

From Latin -atus 'id.' From Proto-Italic *-ātos. From Proto-Indo-European *-eh2-tós 'id.' The first element *-eh2- is a thematic vowel; the second element *-tós is an adjective-forming suffix.

Romance: Western Vulgar Latin: Asturian -áu, Portuguese -ado, Galician -ado, Catalan -at, French , Italian -ato; Eastern Vulgar Latin: Romanian -at.
adónde (Adverb) "where"

From Spanish a and donde.
aero- Prefix "air"

From Ancient Greek ἀέρος ‎(aéros), genitive of ᾱ̓ήρ (aér) "air." From Proto-Indo-European *h2eu̯s-er- 'id.' See also aire.
aeropuerto m. (Noun) "airport"

20th cent. neologism from aero- and puerto, a calque of French aéroport.