Etymology

To be taken with red hand in ancient times was to be caught in the act, like a murderer with his hands red with his victim's blood. The use of red hand in this sense goes back to 15th-century Scotland and Scottish law. Sir Walter Scott's Ivanhoe (1819) contains the first recorded use of taken red-handed for someone apprehended in the act of committing a crime. The expression subsequently became more common as caught red-handed.

Adverb

1. In the act of wrongdoing.

   His mother caught him red-handed, reaching into the cookie jar.