The meaning is understood to be caught committing a crime such as murder or poaching with blood on your hands. There was a recent family discussion that there was little doubt where the expression originated, because it must refer to a murderer having blood on his hands. A little research found the answer might not be so uncomplicated. There is a myth about a boat race to the shores of the northern Irish province of Ulster in which the winner of the race would be the ruler. One contestant cut off his hand and threw it to the shore to guarantee his win. The flag of the province has a red hand on a white shield in the center of the flag. Much of the literature does indicate that the meaning is being “caught in the act,” but there are some other theories. Some believe the expression originated in the Indus Valley where a thief’s guilt or innocence was determined by placing his hand on the red-hot heated blade of an axe. Another theory is that the Japanese would brush the sap of poison ivy on money, which would cause the hand of a thief to break into a red rash. There was no explanation for what people did when they had to use the money. I think I’ll go with the blood-on-the-hands explanation, although the other ideas are interesting.