Green’s Dictionary of Slang

chattermag n.

[SE chatter + mag n.2 (1)]

1. a gossip, a chatterbox.

W. Barnes Poems of Rural Life 293: Chattermag, a chattering magpie, a chatterbox, a much-talking woman .
[UK]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.

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