Avada Kedavra, otherwise known as the Killing Curse, is one of the three Unforgivable Curses in Harry Potter's Wizarding World.
One Reddit user named jodatoufin recently posed an interesting theory about the possible connection between Avada Kedavra and a similar sounding magical phrase in the Muggle world: Abracadabra.
This fan thinks the phrase alludes to a deep, dark history between Muggles and Wizards, so much so that, "even after the idea of wizards faded into mythology, among muggles the killing curse became a thing of folklore."
The Huffington Post also dug a little deeper, revealing the chances of this theory being accurate:
It certainly seems plausible. Rowling stated at the 2004 Edinburgh Book Festival that she based the killing curse off "Abracadabra." In her series, we know that the International Statute of Wizarding Secrecy was enacted in 1692, when muggle-wizard relations were at an extreme low (and the Salem witch trials were beginning). Perhaps muggles' persecution of wizards at this time led some to extreme action -- using an Unforgivable curse. Word of the deadly phrase would have traveled.