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1.
During the act of unprotected coitus, immediately post ejaculation the male holds his partner is a pseudo-snuggle until the sensation of impending urination strikes. While the penis remains inside the vagina/anus the male relieves himself, allowing his urine stream to flow freely, flushing out any seminal deposit and filling the body cavity with urine.

Note: When the flush'n'fill leads to a aggressive yeast infection, it is called the sourdough croissant.
Don't worry about getting knocked up, I gave you a flush'n'fill, though you might smell like a pickle jar now. Warn me if you develop a sourdough croissant.
by Jedediah Hornswaggle September 21, 2010