IPA: /nɒnˈfrɪvələs/
KK: /nɑnˈfrɪvələs/
Something that is serious and not silly or trivial.
The committee made a nonfrivolous decision regarding the budget.
Nonfrivolous → It is formed from "non-" (meaning not) and "frivolous" (from Latin *frivolus*, meaning worthless or trivial). The word "nonfrivolous" means not trivial or not lacking in seriousness.
Think of 'not' ('non-') combined with 'worthless' ('frivolus') to remember that 'nonfrivolous' means something that is serious and not trivial.