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1.
Usually takes place after a heavy night of drinking. This is where the female will stop at nothing and is desperate to have sex. Completely neglecting the fact that you may have whiskey dick, she vigorous "mashes" her vagina into your penis in hopes of stimulating a flaccid erection
"Man, Dana was mashing potatoes all night long"
by tyson86 August 08, 2007
 
2.
A euphemism for masturbation
"John, what have you been doing your forearms are massive?"
"Oh you know just mashing potatoes"
by Crystalgeek March 21, 2013
 
3.
The act of stuffing a potato into a woman's vagina and having sex with her until it is mashed. Eating the potatoes is then optional.
I was mashing potatoes with your mom last night.
by MajorRaze May 27, 2011