A logical assumption is that ‘marshal’ is related to ‘martial,’ but the resemblance is purely coincidental. Although most French words are derived from Latin, a few result from the 3rd-century Germanic occupation of France, and the early French ‘mareschal’ is one such word. ‘Mareschal’ came from Old High German ‘marahscalc,’ formed by combining ‘marah’ (horse) and ‘scalc’ (servant). ‘Mareschal’ originally meant ‘horse servant,’ but by the time it was borrowed into Middle English in the 13th century, it described a French high royal official. English applied the word to a similar position, but it eventually came to have other meanings. By contrast, ‘martial’ derives from ‘Mars,’ the Latin name for the god of war, and is completely unrelated.
an officer of the highest rank in some military forces