IPA: //ʌnˈplɔːzəbl//
KK: /ʌnˈplɔːzəbl/
Not believable or unlikely to be true.
The idea that aliens built the pyramids is considered unplausible by most scientists.
Unplausible → It is formed from "un-" (meaning not) and "plausible" (from Latin "plausibilis", meaning deserving applause or approval). The word "unplausible" means not deserving approval or not credible.
Think of 'not' ('un-') and something that is 'worthy of approval' ('plausible') — that's why unplausible means something that is not credible.