RichThesaurus
inexpugnable
vs
never-say-die
inexpugnable
adj
Impossible to eliminate or destroy; impregnable.
never-say-die
adj
That does not give up in despair; indomitable.
How often have the words
inexpugnable
and
never-say-die
occurred in a corpus of books?
(source:
Google Ngram Viewer
)