IPA: /ˌɪrɪˈzɒljʊbəl/
KK: /ɪˈrɛzəˌluːbəl/
Something that cannot be solved or explained.
The team faced an irresoluble problem that left them frustrated.
Irresoluble → It is formed from "in-" (meaning not) and "resolubilis" (from Latin, meaning able to be resolved or loosened). The word "irresoluble" means not able to be resolved or loosened, indicating a state of uncertainty or indecision.
Think of something that cannot be 'resolved' ('resolubilis') — the 'in-' prefix means 'not', so 'irresoluble' describes a situation that remains unresolved.