What is the origin of the saying, "Caught with your pants down?"

From "A Hot On Ice & Other Curious Expressions: The origin & development of the pungent & colorul phrases we all use" by Charles Earle Funk.

"The figurative meaning is, of course, to be taken completely by surprise; to be wholly unprepared. But the literal meaning, I think, takes us back to the days when white frontiersmen were exposed to peril if their muskets were not actually at hand. Even at the risk of death, the wants of nature must be met, and in such circumstances a lurking Indian would indeed catch one in a state of embarrassing unpreparedness."

There are a few theories.

1. Marcus Antoninus was on a journey when he had to relive himself. His guard gave him privacy and while he was relieving himseld, Julius Martailis rushed up to him and killed him.

2. Frontiersmen couldn't go anywhere without danger. But man is bound by his bowels, and weapons were taken along when they had to go so he couldn't be caught unprepared.

3. A cheating husband would be caught without any pants.

http://grahams-random-ramblings.blogspot.com/2008/09/meaning-and-origin-of-caught-with-his.html

The Emperor Caracalla-a tyrant remembered for slaying his brother and building the extravagant Baths of Caracalla-was murdered by his own guards while he was relieving himself. That may be where the phrase "caught with your pants down" comes from."

http://ancienthistory.about.com/b/2005/08/19/caught-with-your-pants-down.htm