IPA: /ˌʌnɪmˈpɔːtənt/
KK: /ʌnɪmˈpɔrtənt/
Not significant or valuable; having little or no importance.
The details of the meeting were unimportant compared to the main topic.
Unimportant → It is formed from "un-" (meaning not) and "important" (from Latin "importans", meaning significant or valuable). The word "unimportant" means not significant or lacking value.
Think of 'not' ('un-') and something that is 'valuable' ('important') — so unimportant means it lacks significance.