A person who frequently makes careless or clumsy mistakes.
From 'blunder' plus the agent suffix '-er,' which forms nouns indicating someone who performs an action. This pattern is productive in English, creating 'runner' from 'run,' 'singer' from 'sing,' and 'blunderer' from 'blunder.'
A 'blunderer' isn't necessarily stupid—often it's just someone in too much of a hurry or not paying close attention, which is why we all blunder sometimes but hopefully aren't full-time blunderers!
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