Furor Teutonicus ("Teutonic Fury") is a Latin phrase referring to the proverbial ferocity of the Teutons, or more generally, of the Germanic tribes of the Roman Empire period.
Generally, the original expression is attributed to the Roman poet Lucan (d. AD 65).
Lucan used the term to describe what he believed to be the outstanding characteristic of the Celtic/Germanic called the Teutones: a mad, merciless, berserk rage in battle.
[1] The Teutons met with the armies of the Roman Republic in the Eastern Alps around 113 BC.
Under the command of the Consul Gnaeus Papirius Carbo, the Romans tried to lure the tribe into a trap, but the Romans underestimated their military potential and lost the Battle of Noreia.