Dating back to around the 1st century CE, the Mouth of Truth is a tall stone disc carved into a humanoid face with hollow holes for eyes and its gaping mouth. The original purpose of the large medallion has been theorized as everything from a ceremonial well cover, to a piece of fountain decoration, to a manhole cover. The face itself has been said to represent a pagan god although exactly which one is up for debate with scholars guessing at everyone from forest god Faunus, to sea god Oceanus, to a local river god.
While the origin is up for debate the one unifying legend surrounding the stone carving is that if one were to stick their hand inside the disc's mouth and tell a lie, the rocky maw would bite the offending hand off. This belief seems to have originated during the Middle Ages when the disc was supposedly used during trials having the accused put their hand in the slot and if found to be untruthful a hidden axeman would lop off the appendage. While this use seems to be apocryphal, the superstition persists to this day.
The Mouth of Truth, which now rests outside the doors of the Santa Maria in Cosmedin church, has been used as a whimsical lie detector in a number of movies and video games, most famously in the 1953 romance, Roman Holiday, in which the carving was a major plot device.