The adjective redhanded, meaning guilty, is a reference to having blood on one’s hands. The term comes from red hand a 15th century Scottish legal term meaning with clear evidence of guilt. From a 1432 Scottish law:
Gif he may be ouretakyn he salbe put in sikkir festines quhil þe law be done on hym...Ande be it red hand it salbe done wtin þat sonne.
(If he may be overtaken he shall be put in secure restraints while the law be done on him...And if it is red hand it shall be done within that day.)
The more modern form redhanded dates to the early 19th century, first appearing in Walter Scott’s 1819 Ivanhoe:
I did but tie one fellow, who was taken redhanded and in the fact, to the horns of a wild stag.
(Source: Oxford English Dictionary, 2nd Edition)
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