red-handed

Etymology:

• From red + handed, likening to a murderer with their hands red with the victim's blood. The phrase to be taken with red hand originally meant "to be caught in the act". The use of red hand in this sense goes back to 15th-century Scotland and Scottish law. Sir Walter Scott's Ivanhoe 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.