I think it would be fair to disallow "hapax" due to obscurity, but it does seem to be a valid word in English: https://www.merriam-webster.com/dictionary/hapax
but it's not because the definition is a link to hapax legomenon. Solely because it is an ellipsis of legomenon. This has made me more frustrated then it should but maybe the creator will take note for next time.
I checked dictionary.com for it and it wasn't found. That site draws from Webster, so.
even on Webster it's definition links to hapax legomenon ... I feel the creator tried too hard and failed miserably.
