The antonym for the word credibility is incredibility, which means lacking credibility or believability.

The three types of credibility for a speech are initial credibility (based on the speaker's reputation and qualifications), derived credibility (built through the speaker's presentation and content), and terminal credibility (final impression of the speaker's credibility after the speech).

The lawyer's strategy was to attack the credibility of the witness.

Credibility means something that is believable.

The quality of being credible; credibleness; as, the credibility of facts; the credibility of witnesses.