The only free and comprehensive online etymological dictionary of the Spanish language
cansar (Verb) "to tire"
11th cent. From Latin campsare "to avoid." Probably borrowed from Ancient Greek κάμψαι (kámpsai) "to be bent," "to be sailed around;" the aorist infinitive of κάμπτειν (kámptein) "to bend," "to curve." Of unknown origin, probably borrowed from a non-Indo-European language. Probably from the same origin of campo.
Asturian cansar, Portuguese cansar, Galician cansar, Catalan cansar, Italian cansare

Penny (2002) notes that cansar is a particularly interesting archaism in Spanish. Because the Romans entered the Iberian peninsula and began the process of Latinization at such an early date (~3rd cent. BCE), the Latin dialect of the peninsula inherited a number of archaic vocabulary terms. Cansar is one such item, not found in Latin literature after the 2nd cent. BCE but evidently preserved in the dialect of Latin to become Spanish.