Thraex. It was the Special Weapon of the Gladiator.
Early Sicas had a sharply curved, single-sided blade, but the later design (the one shown in Deadliest Warrior) had a straight, double-edged blade with the upper third of the blade bent backwards at a 45° angle.
The Sica's angled blade was designed to reach around the shields of opponents, which was necessary since the Thraex's typical opponent, the murmillo class of gladiator, carried the large scutum shield of Roman legionaries. With the tip pointing up, the Sica would often catch on the bottom of opponent's helmets, allowing the user to force the blade upwards through the jaw and into the brain.
Length: 30 inches
Weight: 2.5 pounds
Composition: Hardened steel