source: 桃矢がケロベロスの存在を疑うGIF画像｜無料GIF画像検索 GIFMAGAZINE 780416

(of a person or their manner) unwilling or unable to believe something:

〈人が〉疑い深い; 〈顔つきなどが〉信じていない, 疑うような

e.g. an incredulous gasp.

incredulousness | inˈkrejələsnəs | noun

See usage at incredible.

16th century: from Latin incredulus (from in- ‘not’ + credulus ‘believing, trusting’, from credere ‘believe’) + -ous.