The only free and comprehensive online etymological dictionary of the Spanish language
campo m. (Noun) "countryside," "field"
10th cent. From Latin campus "field" but metaphorically a field (for an event like a battle or debate). A similar development occurs in English phrases like "playing field" or "battlefield." From Proto-Italic *kampo- 'field.' Of unknown origin, probably a substrate source.
The original sense in Latin of a treeless field is even better preserved in campa "empty field." Also the origin of myriad toponyms through Spain and the Spanish-speaking world. As a surname in Campo, Campos, del Campo, Cámpiz, it was given to families known for their homes in the countryside.
Indo-European
Romance
Asturian campu, Portuguese campo, Galician campo, Catalan camp, French champ, Italian camp, Aromanian cãmpu, Romanian câmp, Sardinian campu
Germanic
Gothic hamfs "mutilated," Old High German hamf "maimed," Old English hāf 'id.'
Balto-Slavic
Lithuanian kam̃pas "corner," kum̃pas "curved," Latvian kùmpt "to be bent," Slavic Old Church Slavonic kǫtъ "corner"
Hellenic
Ancient Greek καμπή (kampé) "bow"

Related to Spanish campeón as a champion fighter was the foremost on the field of battle.