Australian slang (possibly used elsewhere).
Used to describe a person who is surprisingly foolish or inept. Not entirely derogatory - moreso a playful taunt.
"Geez, Joe, you're such a shocker!!"
(after Joe did something stupid)
"Wow, MJ had a real shocker in that game - he only scored 11 points!"
(a woeful sporting performance will often get this call)
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