It originates from a time when men wore knickerbockers. In those days, underpants or shorts etc. were virtually unheard of. This meant that whatever was in there was at the mercy of the coarse woolly tweed, which, due to exertion of some sort would bunch up and wreak havoc on tender parts residing within, causing a marked deterioration in the mood of the wearer/owner.
Thank god for progress.


Chris