Don’t take a facetious comment seriously because it’s supposed to be funny. Anything facetious is a joke. If you've just won a hotdog-eating contest and someone asks if you'd like to go out for burgers, they’re probably being facetious.
If you forget how to spell facetious, notice that it has all five vowels in a row. The word facetious comes from the French facétie, “joke,” and it has come to describe a joke with a little drop of sarcasm. It used to simply mean “funny and witty,” but now it often implies that someone is being inappropriately funny about a serious topic.