Discussion Forum—A Way with Words, a fun radio show and podcast about language

I just checked a list of the 300 most commonly used adjectives at: http://www.examples-help.org.uk/parts-of-speech/list-of-adjectives.htm

As you can see, not a single one ends with an "x" and "manx" wasn't even listed, no doubt because it's not that commonly used. I'd never heard it before. Racked my brain and couldn't think of another solid example. Perhaps "six" would count as an adjective in "We saw the six people arrive." Perhaps not. And it's not really taking an "x" ending anyway, which is what you were looking for.

I have found this particular instance described as a unique and irregular formation of a demonym: Isle of Man → Manx.