First, I don't wear knickers (interesting choice of words though). I am quite relaxed, listening to a little music, sipping a nice glass of merlot; and lurking on FR. What makes you think otherwise?
W.K.
The term is used to describe a situation wherein someone appears to be inappropriately belligerent or bellicose relative to the discussion at hand. It can apply to men or women.
Now, if you were in west Texas, it would go something like "Sugar, what have you got your panties in a knot over?" I'm sure there are other regionalisms that would describe this situation - these are just two that I would use.