A person who shakes their head, or something so surprising or disappointing that it causes people to shake their heads in disbelief.
From 'headshake' plus the agentive suffix '-er' (one who performs an action). This informal term emerged in modern English.
In sports, a 'headshaker' is what announcers call a stunning upset or unbelievable mistake—when something happens that's so shocking your brain literally wants to deny it by shaking your head.
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