Something that surprises or upsets people greatly; a shocking revelation or unexpected event that startles everyone.
From 'shock' (a sudden disturbance of emotions) plus '-er' (one who does). The suffix '-er' creates nouns from verbs, making 'shocker' mean 'something that shocks' in the 1900s.
The suffix '-er' is so productive in English that we can add it to almost any verb to create a noun—if something is new and unsettling, English speakers will eventually call it a 'shocker'!
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