An old horse or a person without teeth; something that works or functions through adhesive qualities.
From 'gum' (the tissue in the mouth, from Old English 'goma') plus the agent suffix '-er.' Also derived from archaic slang for an old, worn-out horse whose teeth have fallen out.
A 'gummer' is a toothless horse—the kind of old, worthless animal that nobody wanted—which is probably why it became slang for anything broken down and useless, which is actually kind of sad when you think about it.
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