Elements describe「fastening, tying down」with「spurting stance」and「many, taut erect rod」within an orifice that when rearranged with「the reason for sheep」comes together to form the age-old human male practice of tupping sheep, and in this case, gang-banging by many taut erect rods until the non-human animal's fleece becomes smeared and pasted with its own feces and human ejaculate slurry, the proverbial BLACKENED SHEEP, or「sheep as a means to an end.」Refers to OVINE ZOOERASTIA, BESTIALITY, and to the societal plague: inconceivable ANIMAL HUSBANDRY, fornication with no chance of conceiving.
　What is conceivable, however, is conceiving a case of the pox: syphilis in humans perhaps caused by tupping sheep and goats, or at the very least being attributable to tupping sheep. Gangrenous, diseased flesh changes color, to black. A human who「depends upon sheep」for a praiseworthy experience will often cause the hind end of the animal to be smeared with blackish paste, and doing so is also a great way to become infected. Clearly, the terms in English related to BLACK SHEEP of the family being regarded as a disgrace and an embarrassment to the rest of the group and so on, were derived from getting the pox while anal-tupping in ovine zooerastia, someone who caused white wool (family name and reputation) to be smeared with black feces: formerly, the term in fact was the 'blackened sheep of the family' which makes more sense socially, genetically, and scatologically.
　The commonly held explanation of 'black wool cannot be dyed' makes no sense, since black garments have also in been demand for millennia, and that color of natural wool works quite nicely…in fact, sheep wool naturally is produced in a wide range of shades and colors other than whitish, but those colors were not disparaged. Rather than being negative, the wool of black sheep may have been prized as it could be made into dark cloth without dyeing 　[　https://en.m.wikipedia.org/wiki/Baa,_Baa,_Black_Sheep 2019-07-14　]　

Modern definitions (that generally disregard history) …excluding politically incorrect concepts and other meanings deemed offensive today; may list only pigeonholed definitions, euphemisms, or meaningless mnemonics)
Unihan extended: a plague or a pestilence of goat (sheep); black goat or sheep, black