For those of you hoping to read of a fantastic mythological reference, or an interesting metaphor that gave rise to this expression, I’m afraid I will have to disappoint you! The origin of this phrase is straight-forward. The “red” refers to blood, the idea being that if you have just performed a murder, you might just still have blood on your hands, which would therefore be literally red. Over time, this expanded to include other crimes as well as murder and the phrase took on a more figurative meaning.
The expression first appears in the form “red-hand”, and it can be found in Scottish legal contexts, such as in the Scottish Acts of Parliament from 1432, and other documents relating to legal matters in Scotland, where it was obviously considered a perfectly acceptable phrase to use in formal situations.
It was predominantly a Scottish expression for a few centuries, until Sir Walter Scott wrote his novel Ivanhoe in 1819, and used it there;
“I did but tie one fellow, who was taken redhanded and in the fact, to the horns of a wild stag.”
This novel served to popularise the expression outside of Scotland.
Looking outside of English, in Polish you would say przyłapać na goracym uczynku, which translates literally as “caught someone in the heat”.
In another Slavic language, Slovene, there are two possibly expressions, one of which has a similar imagery to the English; imeti krvave roke, which means “one has blooded hands”, or alternatively ujet na delu, meaning “caught at work”, much like “caught in the act”.
In French, just as in Italian, you would be caught with your hands in the bag: être pris la main dans le sac.
Spanish has an expression that brings bread to mind: lo/la agarraron con las manos en la masa; “he/she was caught with hands on the dough”
In German you might say auf frischer Tat ertappt worden, or “be surprised in the act”.
Over to Welsh now, where we have a particularly interesting note. Firstly, there is the expression dal rhywun ar y weithred, which literally means “to catch someone in the act”. But this phrase is almost incidental in our Welsh note today. Far more intriguing is the word llofrudd. Today, this word means “murderer”. Let’s take a look at the components of the word: llof + rhudd. The first part, llof, is an old word for hand (the modern word being “llaw“). The second part, rhudd, as you have probably guessed, means “red”. Put it together and you get “red-handed”. One has to wonder about the ultimate origins of the phrase in English, prior to it finding its way into the Scottish idiom.
Another Celtic language, Irish, has the phrase breith maol ar dhuine.
I am quite fond of the equivalent phrase in Greek; πιάνω κάποιον στα πράσα (piano kapion sta prasa), which literally means “catch someone in the leeks”!