Emily Dickinson Lexicon

MYR'MI-DON, n. [Gr. μυρμηδων, a multitude of ants; W. myr; qu. so called from their numbers or from their industry.]

Primarily, the Myrmidons are said to have been a people on the borders of Thessaly, who accompanied Achilles to the war against Troy. Hence the name came to signify soldier of a rough character, a desperate soldier or ruffian.