fallar
/faˈʝaɾ/ · A2 · verb
to fail
fallar = English fail with a Spanish -ar ending; both from Latin fallere
Latin fallere (to deceive, disappoint) gave English fail, fault and false, and Spanish fallar and falta. The disappoint sense survives directly: fallarle a alguien is to let someone down. Same root, so swap fail for fall- and add -ar.