third month of our year, first month of the ancient Roman calendar, c. 1200, from Anglo-French marche, Old French marz, from Latin Martius (mensis) "(month) of Mars," from Mars (genitive Martis). The Latin word also is the source of Spanish marzo, Portuguese março, Italian marzo, German März, Dutch Maart, Danish Marts, etc.
Replaced Old English hreðmonaþ, the first part of which is of uncertain meaning, perhaps from hræd "quick, nimble, ready, active, alert, prompt." Another name for it was Lide, Lyde (c.1300), from Old English hlyda, which is perhaps literally "noisy" and related to hlud "loud" (see loud). This fell from general use 14c. but survived into 19c. in dialect.
For March hare, proverbial type of madness, see mad (adj.). The proverb about coming in like a lion and going out like a lamb is since 1630s. March weather has been figurative of changeableness since mid-15c.