Unassailable (adjective)

Not able to be challenged or defeated; unassailable.

Origin:

From the word assail meaning "to attack" and the prefix un- meaning "not".

Examples:

1. The city's defenses were unassailable.
2. The company's market position was unassailable.
3. The evidence against him was unassailable.
4. The argument was unassailable, leaving the opponent speechless.
5. The leader's popularity was unassailable.