E. Cobham Brewer 1810–1897. Dictionary of Phrase and Fable. 1898.
Mag.
What a mag you are! jabberer, hence to chatter like a magpie. Mag is a contraction of magpie. The French have a famous word, “caquct-bon-bec.” We call a prating man or woman “a mag.” (See MAGPIE.)
