Caught red-handed

This one comes from Scots law. From at least the 15th century, "red hand" appears in Scottish legal texts describing a criminal (usually a poacher or a murderer) caught with blood literally still on their hands, which was treated as decisive proof of guilt. The phrase moved into general English usage in the 19th century, most notably popularised by Sir Walter Scott (again) in his 1819 novel Ivanhoe.