'em dash' originates from English printing terminology; specifically the element 'em' is the typographic unit named after the letter 'M' (the width of the letter 'M'), and 'dash' originates from Middle English 'dashe' meaning 'a stroke' or 'strike'.

'dash' changed from the Middle English word 'dashe' and developed into the modern English 'dash'; the compound term 'em dash' arose in typesetting to describe a dash roughly the width of an 'M', and later became the name of the punctuation mark '—'.

Initially the term referred to a typographical length (an 'em' being the width of the letter 'M') and 'dash' to a stroke; over time the compound came to denote specifically the long punctuation mark used for breaks and ranges.