A semi-chewed piece of food (it can be any food but most commonly a sandwich) which is spat out onto the ground. Someone may wish to birth a mandrake for several reasons: an unpleasant tasting mouthful, an unwanted object e.g. a fly may have entered the eater’s mouth without consent or even just for fun.
The word originates from North Yorkshire where a boy named Caleb first gave birth to a mandrake outside U07 at South Craven School.
Oh look at that mandrake on the floor, it looks just like the insides of a Big Mac.