"A dash derives from "to dash," to shatter, strike violently, to throw suddenly or violently, hence to throw carelessly in or on, hence to write carelessly or suddenly, to add or insert suddenly or carelessly to or in the page. "To dash" comes from Middle English daschen, itself probably from Scandinavian-compare Danish daske, to beat, to strike. Ultimately the word is-rather obviously-echoic."

Source : Eric Partridge (2003). “You Have a Point There: A Guide to Punctuation and Its Allies”, p.70, Routledge

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