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.

A person who gets the vaccine and and flaunts it will high end shopping, trips, and parties

The exact moment of seeing a person doing illegal acts.

Finger another guys girl while she's in her period, and he walks in the room.

When a young Mexican boy reaches the bottom of his most recent bag of Hot Cheetos and the spicy Cheeto powder coats his little brown digits.