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1.
Similar to "gotta jet" or "gotta bounce", this phrase means to leave somewhere very quickly with speed matching that of prom dress removal at a post-prom party.
Talk to ya later, I'm off like a prom dress.
by jonlindg October 04, 2005
 
2.
departing, or about to depart
As soon as he finished his meal he was off like a prom dress.
by The Return of Light Joker September 03, 2011