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The art of one party performing fellatio on another party in the bath, whilst a friend, colleague or close associate of either of the bathing individuals excretes using a toilet situated in the same room as the aforementioned bath.
Charlotte gave Ian a blathshib in the bath tub, whilst their close friend Timothy took a dump on the ajacent toilet facility.

"I say honey, my brother is in the bathroom taking a shit - fancy drawing a bath and giving me a blathshib?"
by *OZZY* August 18, 2008
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