Unthinkable (adjective)

1. Impossible or almost impossible to imagine or consider.
2. Impossible or extremely unlikely to happen.

Origin:

Early 19th century: from un-1 (expressing negation) + thinkable.

Examples:

1. The possibility of losing the championship was unthinkable for the team.
2. The outcome of the election was unthinkable for many people.
3. The thought of not seeing her again was unthinkable.
4. The destruction of the ancient monument was unthinkable.
5. The failure of the project was unthinkable after so much hard work.