IPA: /ˌʌn.əˈsɜː.tɪv.li/
KK: /ʌnəˈsɜrtɪvli/
In a manner that lacks confidence or assertiveness; not forcefully or confidently expressing oneself.
She spoke unassertively during the meeting, making it hard for others to understand her point of view.
Unassertively → It is formed from "un-" (meaning not) and "assertive" (from Latin "assertivus", meaning inclined to assert or affirm) and "-ly" (meaning in a manner). The word "unassertively" means in a manner that is not inclined to assert or affirm oneself.
Think of 'not' ('un-') being combined with someone who is 'affirming' ('assertive') — so when someone acts 'unassertively', they are not affirming themselves.