I wouldn't put too much weight on the fact that few people got the word, TRex. I think chignon is a hard word to spot, even if you know it. I didn't get it in that puzzle though I did know the word, vaguely.
Nevertheless I feel it's the sort of word that a fair number of players wouldn't know. I presumed that it's a rather old-fashioned word, but things seem a little more complicated. It's true that the word's pinnacle of usage was around 1870, but it went into a significant decline after that, until the 21st century. Looking at both the Google ngram viewer and the Corpus of Historical American Usage, it appears that the word chignon has been used more frequently during the past 20 years than at any time in the 20th century. Quite a lot of those occurrences were in fiction, so it might be that an increase in historical fiction writing has as much to do with the surge in the word's usage as a revival of the hairstyle. However I did see a few articles from the past few years with headlines like "The Chignon Is Making a Major Comeback, Here's How to Wear It".
At any rate, I don't think the word is likely to be as universally well known as necessary to qualify as a common word, so I'll make it rare from now on.
