Page "M-theory" Paragraph 21

When Edward Witten named M-theory, he did not specify what the M stood for — perhaps because the nascent theory was not fully defined.

Some, including Sheldon Glashow, speculate that Witten chose the letter because it resembles an inverted W. According to Witten, " M can stand variously for ' magic ', ' mystery ', or ' matrix ', according to one's taste.