This is another word I was unfamiliar with, although I have encountered the idea that some forms of stress might be beneficial - I was amused some years ago to see a book title, The Joy of Stress.
We can identify the origin of the word eustress with some certainty: Wiktionary tells us it was first used in a 1975 article by endocrinologist Hans Selye. The word formation is based on the pair of prefixes eu-, meaning good, well, etc, and dys- or dis- meaning ill or bad. So, eustress means good stress and distress means bad stress.
Etymologically, this is a bit dubious, because distress is not derived from dis- + stress. If anything, it's the other way around. Stress may have been derived from distress, by the process of aphesis, the dropping of an unstressed (!) syllable from the front of a word.
Actually, it's not easy to come up with a pair of words to illustrate eu- vs dys-/dis-. In almost every case, one of the words is very obscure, if not a total nonce word. For example, euphemism is quite well known, but dysphemism, the deliberate use of unpleasant terms, is far less familiar. Eulexic is one of the grades of members of this forum - Birdy is currently one. But it is pretty much a made up word, on the pattern of dyslexic. Perhaps the best known pair that sound like they follow this pattern are utopia and dystopia. But of course, utopia was never an eu- word. It means simply "nowhere".
Anyhow, back to eustress. It can be found in a few general dictionaries, such as Dictionary.com and the New Oxford American Dictionary. And the word is used, not only in professional writing, but from time to time in general newspapers, management magazines, writings on sports, yoga, etc.
So I'm satisfied that eustress ought to be allowed in Chihuahua.
