Mons Meg. An old-fashioned piece of artillery in the castle of Edinburgh, made at Mons, in Flanders. It was considered a palladium by the Scotch. (See Long Meg.)
“Sent awa' our crown, and our sword, and our sceptre, and Mons Meg to be keepit by thae English ... in the Tower of London [N.B. It was restored in 1828].” —Scott: Rob Roy, chap, xxvii.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894