This word is acceptable for play in the UK dictionary that is being used in the following games:

• Lexulous
• Wordscraper
• Wordosaur

n. The state or condition of being unyielding.

n. The character or state of being unyielding; obstinacy; firmness.

n. resoluteness by virtue of being unyielding and inflexible

"Their fears had vanished, but the sand, receiving now their undivided attention, multiplied its unyieldingness."

resolve    firmness    resolution    resoluteness    firmness of purpose