Impregnable (adjective)

1. Not able to be taken by force; invulnerable.
2. Not able to be defeated, overcome, or penetrated.
3. Not able to be affected or influenced.

Origin:

From the latin in- (not) + praegnans (pregnant) + -able (an adjective suffix).

Examples:

1. The castle was considered impregnable due to its strong walls and strategic location.
2. The fortress was impregnable to the enemy's attacks.
3. The impregnable nature of the encryption made it impossible to crack.
4. The athlete's impregnable mental strength helped him to win the match.
5. The company's impregnable market position made it difficult for competitors to enter the market.