A person or animal that chatters; someone who talks excessively or makes rapid chattering sounds.
From chatter + -er (agent noun suffix meaning 'one who does'). The suffix -er transforms a verb into a person or thing that performs that action.
Interestingly, 'chatterer' is also the name of a group of South American birds that make distinctive clicking and chattering sounds, showing how the same word describes both human talkers and actual animal soundmakers!
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