Where does the word 'Scab' come from?

[JonRB]

Where does 'Scab' come from... as in someone that crosses a picket line? Why are they called Scab?

[Whickerman]

from Wikipedia:

People hired to replace striking workers are often derogatively termed scabs by those in favour of the strike. The terms strike-breaker, blackleg, and scab labour are also used. Trade unionists also use the epithet "scab" to refer to workers who are willing to accept terms that union workers have rejected and interfere with the strike action. Some say that the word comes from the idea that the "scabs" are covering a wound. However, "scab" was an old-fashioned English insult. An older word is "blackleg" and this is found in the old folk song, Blackleg Miner, which has been sung by many groups.

[Whickerman]

...and from Word Detective:

Chances are that when most of us hear the word "scab," we think of its most common sense, that of the crust that forms on top of a wound. That was the original meaning of "scab," which we borrowed directly from the Old Norse word "skabbr" back in the 13th century.

Although any doctor will tell you that a scab is a good thing, since it protects a wound while it is healing, by about 1590 we were using "scab" to mean "a low or despicable person." The logic of this derogatory sense is not entirely clear. It most likely stems from the implication that such a scoundrel might well be afflicted with syphilis, which in its advanced stages causes a "scabby" skin condition. Incidentally, "sceabb," an Old English word related to "scab," eventually became "shab," which originally meant "covered with scabs," but which we use today only in the form "shabby" to mean "run down."

Since "scab" already was being used to mean "lowlife creep," it's not surprising that by the late 1700s it was being applied to any worker who refused to join an organized trade union movement. As one contemporary source explained in 1792, "What is a scab? He is to his trade what a traitor is to his country.... He first sells the journeymen, and is himself afterwards sold in his turn by the masters, till at last he is despised by both and deserted by all."

By the 19th century, "scab" was being used, primarily in the U.S., to mean a worker willing to cross picket lines to replace a striking worker. The great unionizing drives of the 1930's then transformed this sense of "scab" from industrial slang into a household word.