ἄπιστος
ápistos
ap'-is-tos
from Α (as a negative particle) and πιστός; (actively) disbelieving, i.e. without Christian faith (specially, a heathen); (passively) untrustworthy (person), or incredible (thing).

KJV translates as: that believeth not, faithless, incredible thing, infidel, unbeliever(ing)