The phrase "to be caught red-handed" means to be caught in the act of doing something wrong, especially something illegal or dishonest. The origin of this phrase dates back to medieval Scotland and has a clear connection to being caught with evidence of guilt. 

The earliest known use of the phrase comes from medieval Scotland. It referred to someone caught with blood on their hands, specifically in the act of committing murder or poaching. 

The term "red-hand" literally meant having blood on one's hands, indicating that the person had been caught in the act of killing or butchering an animal illegally. 

The phrase appears in Scottish legal texts. For example, the Scottish Acts of Parliament from the 15th century use "red-hand" to describe being caught in the act of a crime. The term evolved to "red-handed" over time, maintaining its original meaning of being caught in the act with clear evidence of wrongdoing. 

The phrase was popularized in English literature by Sir Walter Scott, a Scottish historical novelist. In his novel "Ivanhoe" (1819), Scott writes: "I did but tie one fellow, who was taken redhanded and in the fact, to the horns of a wild stag." Scott's use of the phrase helped cement its place in the English language and broaden its use beyond its original legal context.