noun: urine emitted from a woman's urethra, which rather than successfully reaching the toilet, instead trickles downward and enters that woman's own vagina (usually by accident).
verb: to get urine in one's own vagina while urinating (usually by accident).
Etymology: contraction of "vaginal backwash".
Connie tried to piss lying down, but the last few drops didn't go far enough and vackwashed into her pussy.
Make sure to douche after vackwashing or you'll get a yeast infection.