IPA: /ˌʌnəˈkʌstəmdli/
KK: /ʌnəˈkʌstəmdli/
In a way that is not familiar or usual; behaving in a manner that is different from what one is used to.
She spoke unaccustomedly, surprising everyone with her sudden confidence.
Unaccustomedly → It is formed from "un-" (meaning not), "accustomed" (from Latin "consuetudo", meaning habit or custom), and "-ly" (meaning in a manner). The word describes doing something in a manner that is not habitual or customary.
Think of 'not' ('un-') doing something that is a 'habit' ('accustomed') — that's why unaccustomedly means doing something in a way that is not usual.