Thick layer of excrement plastered across the back of a toilet bowl following high-velocity bowel movements. Gravity and geometry conspire to fashion this matter into a beard-shaped carpet of filth.
Some beards can survive multiple flushes, leading to the ultimate ignobility of cleaning up someone else's beard to avoid implication.
Also known as a "manural".
Oh man, I just went to the toilet and some fellow had left a chocolate beard in there.