IPA: //ʌnˈwɔːrəntəbl//
KK: /ʌnˈwɔːrəntəbl/
Something that cannot be justified or excused; it is unreasonable or unacceptable.
The manager's unwarrantable actions led to a loss of trust among the team.
Unwarrantable → It is formed from "un-" (meaning not) and "warrantable" (from Old French *garantir*, meaning to guarantee or assure, with the suffix "-able" meaning capable of). The word "unwarrantable" means not capable of being guaranteed or justified.
Think of 'not' ('un-') being added to something that can be 'guaranteed' ('warrantable') — that's why unwarrantable means something that cannot be justified.