* A misspelling: Perhaps you meant a similar word like "mattness," which refers to the quality of being dull or flat.
* A regional dialect word: It could be a term used in a specific region or community.
* A made-up word: Someone might have invented the term for a particular purpose.
To understand the meaning, please provide more context.
For example:
* Where did you encounter the word?
* What was the sentence or situation in which it was used?
With more information, I can help you figure out what "matiness" might mean.