Catch someone with their pants down

Origin

The vivid, slightly vulgar image of someone literally caught with their trousers around their ankles, exposed and utterly helpless, is the direct source of this phrase. This physical predicament, dating back to simple acts of nature or moments of undress, became a potent metaphor for any situation where one is discovered unprepared, vulnerable, or in a compromising position. It gained significant traction and widespread use in the 20th century, particularly within American English, as a punchy and memorable way to describe that sudden, embarrassing moment of being caught off guard, much like the unexpected exposure of one's rear.