adj. Derived from the term "bloody-hand", which was one of four kinds of offences in the king's forest, by which the offender is supposed to have killed a deer. In Scotland, in such like crimes they say "Taken in the fact, or with the red hand".
- Thomas Blount's Law Dictionary and Glossary, 1717
In modern times, this term refers to catching someone in the act of doing something they aren't supposed to do.
Hah! I knew it was you stealing the cookies. I caught you red handed!