A shit stain on the underwear usually caused by digging fingers into the ass to scratch out an uncontrolable salty peanut (See salty peanut). Also known as a skid mark.
As she folded the laundry she noticed a large baconstrip in her boyfriend's underwear.
or
He decided to not buy white underwear anymore, because of his baconstrip making problem.
The bacon strip is the ridge that runs along the underside of a mans testicles and up between the balls themselves to join the base of the penis.
Esteban: "hey Helen can you bring me a band aid, I just nicked my bacon strip while I was trying to shave my boys and the gooch again, theres blood everywhere!"