Under cover of a thunderstorm which broke over them they successfully attacked the Marcomanni. (See Legion, Theban Legion.)
This is a mere legend of no historic value. The legion was so called at least a century before the reign of Aurelius; probably because it bore on its shields or ensigns a representation of Jupiter Tonans.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894