Colosseum (or is that coliseum?), arena, circus, muscle, cloaca, and companion have Latin origins that may surprise you!
Today's topic is some common English words that have not so obvious origins in Latin. These words have perhaps been traveling incognito, their identities concealed. In case you aren't familiar with that word, it means “unknown” and it comes from the negative prefix -in and a form of the Latin verb “to get to know.” Stick around to get to know some interesting word history.
Most of you probably know that gladiators fought to the death in venues like the Colosseum, “the largest amphitheater in the Roman world,” according to History.com. The official name of this stadium where men and beasts clashed was Amphitheatrum Flavium, and this amphitheater was so named because emperors of the Flavian dynasty built it. 
The landmark in Rome is usually spelled capital C-o-l-o-s-s-e-u-m, but the spelling c-o-l-i-s-e-u-m is often used as well. Coliseum generally refers to a large venue that hosts entertainers, but it can also be spelled the other way! Take your pick. Two related words are colossal, an adjective meaning “big,” and colossus, a noun that refers to a large statue or anything enormous.
Interestingly, according to the National Geographic website, the Flavian Amphitheater became known as the Colosseum because of “the more than 100-foot-tall bronze statue of Nero depicted as the sun god—the Colossus Neronis—that once loomed over the valley.”
It wasn't so obvious that the word coliseum originated from “big statue.” Even more incognito are the origins of arena, which we all know means a central stage or ring used for sports or other entertainment. If you're familiar with Spanish, you may have figured it out, since arena means “sand.” And that's exactly where the English word arena comes from: a book titled As the Romans Did explains that places like the Colosseum were “covered with sand to soak up the blood.” 
The Romans loved their blood sports, and more blood was spilled at another venue, the Circus Maximus. The largest racetrack in Rome, it held 250,000 spectators,  who flocked there to watch chariot races and gladiator fights. An interesting book about Latin words and phrases—Cave Canem, which means “Beware of the Dog”—states, “The name circus referred to the round shape of the building where chariot races, horse races, and battles were the main exhibits.” 
Modern circuses feature acrobats and elephants, and so did the ancient Circus Maximus. According to an online guide to Italy, “To add variety to events, during the intervals between races they put on acrobatics or fights between exotic animals.”
English words that are related to the round shape of the circus are circle and circuit,  and one modern meaning of the word circus is “an open circle, square, or plaza where several streets converge.” One example is London's Piccadilly Circus, the junction of Regent Street and Piccadilly Street.