Is it embarrassing to own a copy of our dictionary of one-letter words?  For Irish Times columnist Frank McNally, definitely so (but he bravely came out all the same in an article entitled To the Power of M: An Irishman's Diary on the Strange Appeal of the Alphanet's 13th Letter):

Far from bosoms, in fact, the original M was a pictogram for water. And according to my Dictionary of One-Letter Words (it’s sad, I know, but I really have one), the writer Victor Hugo noted that it could also visually represent mountains “or a camp with tents pitched in pairs”.