When idioms are built on metaphors of daily life, and those metaphors die because we no longer live as we once did, those idioms stop being understood. I've noticed of late in both online and printed publications, references to someone being "died in the wool." What they meant, of course, was the other "dye," the one related coloring cloth. Dyed in the wool means "Thoroughgoing; out-and-out: a dyed-in-the-wool populist." Dyed in the wool is almost always used in terms of politics; Kennedy was a dyed in the wool Democrat. But it can be used for other fields as well; I'm a dyed-in-the-wool Macintosh fan. The idiom is derived from the practice of dying wool that is clean, and has been combed or "carded," to remove tangles and bits of trash, but not yet spun into yarn. Wool dyed in this way tends to be more thoroughly, permanently colored.

Historically speaking, the phrase dyed in the wool is one that was used first in England, but which is now much more common in the U.S. The political and religious writer Hooker is the first attested user according to the OED; in 1597 Richard Hooker used the phrase in his Laws of Ecclesiastical Polity "Children as it were in the Wooll of their infancie died with hardnesse may neuer afterwards change colour" (V. lxxii. S18). In an odd way, the use of "black sheep" to refer to "A member of a family or other group who is considered undesirable or disreputable" is related. Black sheep, while considered good omens by shepherds, were less than favorably viewed because their wool was difficult to dye.

We still have a fair number of "sheepish idoms" in English. We talk about a "Bellwether day," in reference to stocks; that's an idiom derived from the wether, or castrated Ram, who leads the flock and wears a bell around his neck. The AHD defines a bellwether as "One that serves as a leader or as a leading indicator of future trends." This too is an idiom we inherited from Britain, but there, oddly, a "bell weather day" is a bad day; Bellwether seems to refer to a negative or downward trend. The OED offers, in additon to the traditional sheep-with-bell definition, the following "a leader; contemptuously: the ring-leader, the worst of the lot." In standard American use, while bellwether refers to a leader or leading trend, the phrase in neutral.

We "count sheep," a thankless, difficult and often boring task when we wish to fall asleep. We refer to someone who is reticent, even shy, as "sheepish," or use "sheepish" to refer to someone who has been "Embarrassed, as by consciousness of a fault," often in the phrase "sheepish grin." Sheep will follow a bellwether over a cliff, or into a canyon, no matter how foolish the action, so the idiom is not without natural cause. Indeed, we still use the phrase "like lambs to the slaughter, " to refer to someone who unquestioningly follows others, no matter how life threatening and dangerous their action may be.