Caught red-handed

Meaning to be discovered in the act whilst doing something illegal.

There are many who have speculated that this term comes from bank robbers having exploding red dye which marks both the loot and them. Others have said it comes from workers in the production of pistachio nuts who would eat all the profits and get caught with red hands.

Unfortunately, it's not that farfetched, and it comes from a time long before exploding dye or red nuts.

In fact, Webbie has it quite nicely put (in bold type for some odd reason) in Red-handed. It first appeared in that form in English in the mid-19th Century, and as an adjective "redhand" was common in Scots (the language of Scotland) since the 15th Century. A murderer caught red-handed still had the blood of his victim on his hands.

According Evan Morris (a.k.a. The Word Detective), the term originated in 15th century England, referring to the blood-stained hands of an individual who hadn't yet found the time to wash clean the evidence of his crime. Interestingly enough, no specific crime is associated with the origin of this word, as it seemed appropriate to describe several common misdeeds, such as homicide, killing livestock or even theft from a butchery.

The term came into common use in the 1700's, and found its way into the Oxford English Dictionary in the 1800's.

Many thanks to The Word Detective, via http://word-detective.com