If you are caught red-handed you are discovered in the middle of committing a crime or doing something wrong. Criminals can be caught red-handed by the police at the scene of a crime or children can be caught red-handed committing some minor misdemeanor like taking food from the fridge in the middle of the night. For many centuries it was not considered to be proof of guilt if someone was found in possession of meat from another man’s animal. If, however, they had the blood of a freshly killed animal on their hands, this was incontrovertible proof of guilt. In those days being caught red-handed usually meant a death sentence. Nowadays, it often merely means being embarrassed.
The expression originated in Scotland around the 15th century. Given the context often used in the earliest references, the phrase ‘red hand’ or ‘redhand’ probably referred to people caught with blood on their hands. The first known documented instance of ‘red hand’ is in the Scottish Acts of Parliament of James I, written in 1432. It subsequently appeared numerous times in Scottish legal proceedings, nearly always referring to someone caught in the act of committing a crime, such as ‘apprehended redhand,’ ‘taken with redhand,’ etc.
The first documented instance of the expression morphing from “red hand” to “red handed” was in the early 19th century work Ivanhoe, written by Sir Walter Scott. Scott was an avid student of Scottish history and folklore, which he relentlessly mined for inspiration in his novel writing. The enormous popularity of his books certainly brought 'red-handed' to a wide audience and, without him, the term might now be long forgotten.
The Red Hand has long been a heraldic and cultural symbol of the northern Irish province of Ulster. One of the many myths explaining its origin is the tale of how, in a boat race in which the first to touch the shore of Ulster was to become the province's ruler, one contestant guaranteed his win by cutting off his hand and throwing it to the shore ahead of his rivals. The potency of the symbol remains and is used in the Ulster flag.