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

• credit from mid 16th century:

  People first used the word credit (ultimately from Latin credere ‘to believe or trust’) to mean ‘belief’ and ‘trustworthiness’. The modern sense developed from the idea of, say, a shopkeeper's trust that a customer will pay for goods at a later time. Credere also gave us creed (Old English), credence (Middle English) , credential (Late Middle English), credible (Late Middle English), and incredulous (late 16th century). You can give credit where credit is due to show that you think someone deserves to be given praise. The earlier form of the saying was ‘honour where honour is due’, a phrase from the Bible, from the Epistle to the Romans: ‘Render therefore to all their dues: tribute to whom tribute is due; custom to whom custom; fear to whom fear; honour to whom honour.’