This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

• Lexulous
• Wordscraper
• Wordosaur

adj. superlative form of firm: most firm.

"Plus: This simple-to-operate machine—with digital display, auto shut-off and nifty jars that let us mark the date we made the yogurt—consistently produced the firmest, best-tasting yogurt."

unshakeable    unswerving    stanch    surer    unshaken