dis ‧ be ‧ lieve /ˌdɪsbəˈliːv, ˌdɪsbɪˈliːv/ BrE AmE verb [intransitive and transitive] formal
[ Word Family: noun : ↑ belief , ↑ disbelief , ↑ believer ; adjective : ↑ believable ≠ ↑ unbelievable , ↑ disbelieving ; verb : ↑ believe ≠ ↑ disbelieve ; adverb : ↑ unbelievably ]
to not believe something or someone ⇨ doubt :
I see no reason to disbelieve him.
—disbelieving adjective