“Caught red handed”, has its origins in
Scotland around the 15th century. Given the context it was often
used in the earliest references, the phrase “red hand” or “redhand”
probably came about referring 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
popped up numerous times in various legal proceedings in Scotland,
nearly always referring to someone caught in the act of committing
some 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.