Person's meaning behind the tattoo: "The spell actually means ‘I await for a protector” in Latin. In the Harry Potter itself, ‘Expecto Patronum” is used to repel Dementors. Dementor is the guard of the Azkaban prison, a dark creature that absorbs your joy and happiness. So, in other words, the spell is used to repel all the bad aura that wants to steal happiness. The unique thing about it is, if you want to cast the spell, you have to think of things that make you happy." though I would never get a tattoo I liked this explanation of the spell.