This idiom means to be caught in the act of doing something wrong or sneaky, typically something minor or mischievous, with clear evidence of guilt. It evokes the image of a child caught stealing cookies, suggesting a playful or relatable transgression, and is used in contexts of petty misdeeds or indiscretions.

The phrase emerged in early 20th-century America, rooted in domestic imagery of children sneaking treats from a cookie jar, a common household item. It appeared in print by the 1920s, with a 1925 *Saturday Evening Post* story using ‘hand in the cookie jar’ to describe a child’s mischief. By the mid-20th century, it was metaphorical, applied to adults caught in minor wrongdoings, as seen in political cartoons and journalism. Its lighthearted, universal imagery ensured its popularity in American and global English.