Green’s Dictionary of Slang

1. a gossip, a chatterbox.

1844		W. Barnes Poems of Rural Life 293: Chattermag, a chattering magpie, a chatterbox, a much-talking woman .
1893		G. Gissing ‘Lou and Liz’ in Keating Working Class Stories of the 1890s (1971) 80: Stop yer jaw Lou! What a chattermag you are!

2. chatter, gossip.

1895

C.W. Scott Apple Orchards 25: Gossip and chattermag, [...] bigotry, and ignorance are all in full blow .