Improbable (adjective)

1. Unlikely to happen or to be true.
2. Not having a high chance of success.
3. Not expected to happen.

Origin:

Late 16th century: from french, or from latin improbabilis, from in- "not" + probabilis "probable".

Examples:

1. It"s improbable that he"ll win the election.
2. An improbable series of events led to his success.
3. It is highly improbable that the project will be completed on time.
4. The improbable story of her survival captivated the audience.
5. The improbable coincidence of the two events happening at the same time was astonishing.