Top Definition
The act of breaching the water with ones ass like a whale breaching for a breath of air.
I gave the life guard at the public swimming pool a rumpback whale and they banned me for the summer.
by Hardly brothers July 27, 2009
The ass-like crack formed by a fat lady wearing a tight bathing suit in the middle of her back.
That lady has a plumber's rumpback whale.
by Hhead August 27, 2013

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