The act of two women (lesbian or not) sitting across from each other with their legs spread open and sideways, allowing for full contact of their vagina to one another. Once in contact, they begin rubbing/grinding themselves against the other's vagina. Not entirely unlike the act of using a two-headed dildo and coming all the way together, only this does not involve a dildo.
My dyke neighbors would probably spend a romantic evening flat cocking each other, since they were too poor to buy a dildo or strap-on.
I can't believe nobody has heard of the term "flat cocking"