The act of talking excessively or gossiping continuously, especially associated with women.
From chattermag (a female chatterbox) + -ing (gerund/present participle suffix). The verbification of the noun extends the meaning to describe the behavior itself.
Chattermagging shows how English creates new verbs from nouns describing people—once you have a word for 'excessive female talker,' it's natural to create a verb form for 'the act of being a chattermag'!
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