The difference between Impregnable and Undefeatable
When used as adjectives, impregnable means too strong to be penetrated, whereas undefeatable means not defeatable.
Undefeatable is also noun with the meaning: something that has an undefeatable quality.
check bellow for the other definitions of Impregnable and Undefeatable
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Impregnable as an adjective (of a fortress, wall, etc., also used figuratively):
Too strong to be penetrated.
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Impregnable as an adjective:
Capable of being impregnated.
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Undefeatable as an adjective:
Not defeatable; impossible to defeat.
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Undefeatable as a noun:
Something that has an undefeatable quality.