Something that chacks or makes chacking sounds; potentially an agent noun from the verb 'chack.'
Derived from 'chack' (verb) with the agent suffix '-er,' which creates nouns meaning 'one who does X' or 'that which does X,' a productive pattern in English since Old English.
The '-er' suffix is incredibly productive in English—you can almost add it to any verb and make a new word: 'jump' becomes 'jumper,' 'teach' becomes 'teacher,' 'chack' becomes 'chacker!'
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