The only free and comprehensive online etymological dictionary of the Spanish language
abajo (Adverb) "down," "below"
First attested c. 1300 but was rarely used until the 15th cent. Formed from Latin a(d)- "at," "toward" and bassus "down" (see a- and bajo respectively).
Also the origin of the surname Abajo, given to Moorish families that lived in the lowlands of Spain.
Asturian (Teberga) abaxu, Portuguese abaixo, Galician abaixo, Catalan abaixo, French à bas