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To flail your arms/legs around wildly. Usually done in excitement or joy, but sometimes done in horror.
Siobhan was so happy when Casper asked her to prom, she promptly started flailspacking and knocked over the nearest vase.

The cannibalistic demon was so gross and terrifying that Mandy started flailspacking in the middle of the theater.
by museFTW April 10, 2010