This idiom means to be caught off-guard, unprepared, or inattentive, often missing an opportunity or being surprised by an event. It suggests a lapse in vigilance, like someone caught sleeping, and is used in contexts of competition, work, or personal oversight.

The phrase likely originated in 16th-century England, tied to the literal idea of being caught asleep. An early use appears in William Tyndale’s 1526 Bible translation: ‘Lest he come suddenly and find you napping.’ By the 19th century, it was metaphorical, with Charles Dickens using it in *Bleak House* (1853) to describe negligence. Its use in sports and military contexts, where alertness is critical, reinforced its popularity in English.