IPA: //nɒnˈsɪəriəs//
KK: /nɑnˈsɪr.i.əs/
Not having a serious attitude or purpose; lighthearted or playful.
His nonserious approach to the project made it difficult to get any work done.
Nonserious → It is formed from "non-" (meaning not) and "serious" (from Latin "serius", meaning grave or important). The word "nonserious" means not grave or important, indicating a lack of seriousness.
Think of 'not' ('non-') combined with something that is 'grave' or 'important' ('serious') — that's why nonserious means something that isn't serious.