Providing credible sources if possible, what is the etymology of 負けず嫌い? Is there any difference in meaning or strength between 負け嫌い and 負けず嫌い?
It says むず connects to an imperfective form of verbs/some auxiliary verbs etc, and it's said that it's a transformation from
auxiliary verb "ん/む"+
verb "す" ("んとす/むとす"). It says "むず" usually has basically the same meaning as "ん/む", but it incorporates extra emphasis.
According to this page, it means something like 負けるだろうことが嫌い, or "hate thinking that you'll lose".