The Roman Emperor Caracalla,later
known as Marcus Antoninus. He was known as one of the bad
ass Emperors round about 200BC. History has it that while he
was on a journey to somewhere important his armed escort
gave him privacy to relieve himself next to the road. A
certain Julius Martailis, who had a grudge against Caracalla took advantage of the opportunity to run forward and kill the Emperor with a single sword stroke. He tried to flee after the assassination but was killed by an arrow from one of the bodyguards of the now deceased Emperor.
A second source relates to the frontiersmen who could go nowhere without exposing themselves to danger. But, when a man has to go, he has to go, and weapons were taken along so that one could not be caught unprepared. It would be very difficult to defend oneself with your trousers around your ankles.
Third one relates to an adulterous husband caught in the act.