From apistos; to be unbelieving, i.e. (transitively) disbelieve, or (by implication) disobey -- believe not.
see GREEK apistos
1. to betray a trust, be unfaithful: 2 Timothy 2:13 (opposed to πιστός μένει); Romans 3:3; (others deny this sense in the N. T.; cf. Morison or Meyer on Romans, the passage cited; Ellicott on 2 Timothy, the passage cited).
2. to have no belief, disbelieve: in the news of Christ's resurrection, Mark 16:11; Luke 24:41; with dative of person, Luke 24:11; in the tidings concerning Jesus the Messiah, Mark 16:16 (opposed to πιστεύω) (so 1 Peter 2:7 T Tr WH); Acts 28:24. (In Greek writings from Homer down.)