Have you ever gotten home from school and really needed an after-school snack to give you enough energy to do your homework? You know your parents would want you to have a piece of fruit, but the chocolate chip cookies in the cookie jar are too tempting.
Just as you sneak your hand into the cookie jar to grab hold of a delicious cookie, you hear the voice behind you asking you what you're doing. Bam! You were caught in the act of sneaking the cookie. In fact, many people would say you were caught red-handed.
To be caught red-handed means that you were caught in the act of doing something wrong, with clear evidence for all to see. Evidence is proof that can be shown to establish that you committed the wrong deed.
For example, let's say you grabbed a cookie and ate it before anyone saw you. If your parents knew there were 12 cookies in the jar to start with and there are now only 11 cookies in the jar, they might have good reason to suspect you ate a cookie. However, without seeing you take and eat the cookie, they would have no evidence to rely upon to support their suspicion. Anyone could have taken that cookie.
The phrase “caught red-handed" reflects this idea of being caught in the act with evidence for all to see. Although its origins aren't completely clear, it appears to have developed in the 1400s in Scotland as a reference to poachers.
If you killed an animal on another person's land and were caught with the blood of that animal on your hands, it was clear you were guilty and had been caught “red-handed." Over time, the phrase became popular in the general context of being caught in the act of wrongdoing. It was popularized for the first time in Sir Walter Scott's novel Ivanhoe in 1819.