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"unyielding" is also used as a crossword clue. If that's what you saw, the answer is often adamant (7 letters).
Definitions
resistant to physical force or pressure
Word origin
From un- + yielding.
Used in a sentence
“unyielding pressure”
“unyielding support”
“I refer rather to the central position the creed of unyiedling will holds in the North.”
Source: Wiktionary, CC BY-SA 4.0
Used as a crossword answer1 curated clues
01“Firm and persistent”10 letters
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