Your bicep is the large muscle on the front of your upper arm, the one that bunches up when you bend your elbow to show your arm. Gyms and everyday English often say bicep for one arm and biceps for both, or just “biceps” either way. Though actually, the older and more careful word is biceps even for one muscle. That is the Latin form, and it is already singular: bi- “two” + -ceps “head,” because the muscle has two heads where it joins near the shoulder. English speakers later cut off the -s and made bicep, the way they sometimes do when they think a word is plural. Doctors and textbooks still prefer biceps. The matching muscle on the back of the arm is the triceps, not a second bicep.