Why do we say "The black sheep of the family"?

Meaning:

A family member who is considered bad/worthless by other members of the family and often marginalized or excluded.

Background:

Many sources say that the origin of the expression black sheep of the family comes from Genesis (the first book of the Jewish Torah and Christian Old Testament), and specifically from the story of Jacob agreeing with his uncle that he would be allowed to keep any speckled or dark colored lambs and goats for his own as payment for tending his uncle's sheep. But, others say this thinking is flawed, firstly because the reference to black sheep is probably an early mistranslation that was then copied into later editions; and secondly, and more fundamentally, even if black sheep were referenced in early versions of the story, the context does not explain the modern use of the term meaning a disreputable member of the family:

"Let me go through all your flocks today and remove from them every speckled or spotted sheep, every dark-colored lamb and every spotted or speckled goat. They will be my wages. And my honesty will testify for me in the future, whenever you check on the wages you have paid me. Any goat in my possession that is not speckled or spotted, or any lamb that is not dark-colored, will be considered stolen." (Genesis 30:32/33, NIV)

According to the Phrase Finder, the first known record of "black sheep"' in a derogatory sense is from the writings of the aptly named Thomas Shepard, a Puritan who emigrated from England to America in 1635. In his evangelical text, The Sincere Convert (1640), he writes:

"Cast out all the Prophane people among us, as drunkards, swearers, whores, lyers, which the Scripture brands for blacke sheepe, and condemnes them in a 100 places."

Strangely, even though the expression black sheep of the family had negative connotations from at least this period, it appears that the black sheep themselves were considered by some as good omens right up until the late 19th century. For example, this reference in The Folk Lore Record (1878):

"We speak figuratively of the one black sheep that is the cause of sorrow in a family; but in its reality it is regarded by the Sussex shepherd as an omen of good luck to his flock."

The rationale behind black sheep being considered good omens is as murky as "black sheep of the family" having a negative connotation. Perhaps they were considered a good omen precisely because of the story of Jacob, who did very well out of his bargain to take the dark colored lambs and goats as payment. One source says that black wool was prized for its color and purchased at a premium by the clergy, but another says that that black sheep were of low value because the wool could not be dyed; and yet another opines that they were essentially worthless because the Church used to claim the black sheep's wool from farmers as taxes (to be made into cassocks and so forth).

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