IPA: //ʌnˈwɔːrəntəbli//
KK: /ʌnˈwɔːrəntəbli/
In a way that cannot be justified or is not warranted; without good reason.
The decision was made unwarrantably, causing confusion among the team.
Unwarrantably → It is formed from "un-" (meaning not), "warrant" (from Old French *garantir*, meaning to guarantee or assure), and "-ably" (meaning in a manner). The word "unwarrantably" means in a manner that is not guaranteed or justified.
Think of 'not' ('un-') guaranteeing something ('warrant') — that's why unwarrantably means acting in a way that is not justified.