(1) A butt plant is a way to kiss the earth without getting down on your hands and knees or having your upper lips come in contact with it.
It may result from a back-foot slide, which is defined elsewhere, possibly, or maybe not. Who can say?
(2) If you notice a butt plant growing near your butt or out of it, it's time for a shower.
Have someone help out by giving you a serious scrubbing too, OK? Weed often and check frequently for telltale regrowth.
Source: How to talk in the woods.