Lit. 'an army, infantry'. The Latin word used of a military expedition, also the army involved in such an expedition. In the 13c, the word was used in conflicts with Scotland; in the 14c, it was used for the 'expeditions' sent against France. From it we have our modern use, e.g. a military exercise. [< Lat. exerceo = to exercise, to train] -
Dictionary of Medieval Terms and Phrases. Christopher Coredon with Ann Williams.