A mild euphemistic exclamation or intensifier, often used humorously or as a stand-in for a stronger word.
From blank + -ety, likely influenced by rhyming slang or playful word-coining tradition. Used as a euphemism since the early 1900s to avoid saying more offensive words while maintaining the emotional effect.
The British basically invented the art of sounding cross without actually swearing—'blankety blank' became a running gag that comedians could perform on BBC radio, and it was funny *because* everyone knew what you really meant to say but pretended you didn't.
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