Around the beginning of the 20th century this idiom, and its variations, started appearing in conversation and print. It's meaning is rather clear - even if I were dead I wouldn't be caught...engaging in that activity, wearing that item, doing that thing, saying that, engaging with that person, etc. ( source )

Being caught red-handed was a way to catch poachers back in the day, the red referring to the blood still on their hands from killing animals. ( source ) There seems to be some other ideas to the origin but this one seems to be the most plausible.   Today the phrase means being caught in the very act, for example the below picture of me when I was a wee one. 😏 When I was a wee one I got into my Grandma's powder puff and used it liberally on myself and covered the bathroom with the stuff. I was caught red-handed and a picture was taken because I was just too cute to scold! :)