I see that it is believed it comes from 1590 meaning a despicable person but could its usage be influenced by the idea that a scab is a temporary measure the body employs over a wound -- the scab worker does not often expect to last a long time in the job because once the strike is settled, the scab worker has little chance of joining the union?
To be clear, does the temporary nature of the scab's employment have anything to do with why the word is used?
Scab has no connection with something being temporary. Its association is with morality.
Scab was originally a disease of some sort. In such a context, it was never a positive thing:
OED:
1.a. Disease of the skin in which pustules or scales are formed: a general term for skin diseases, but sometimes spec. = itch or scabies (also, dry scab), ringworm or tinea, syphilis; wet scab, eczema.
c1250 Si lepre [signefieþ] þo sennen, þet scab bi-tokned þo litle sennen. Kent. Serm. in Old English Miscellany 31
With less negative force it then developed the current medical meaning of
The crust which forms over a wound or sore during cicatrization.
c1400 Anoynte al his heed..til al þe scabbis þerof be wel tobroke. Lanfranc's Cirurgie
It then developed a figurative meaning (Not unreasonably as disease and other infirmities were said to be punishment for offending God.)
1.b.† figurative. Applied to moral or spiritual disease.
?1529 This is the great scabbe why they will not let the newe testament go a brode yn your moder tong. S. Fish, Supplicacyon for Beggers.
At, or close to, the same time, we have the application to people of dubious moral character - as found in the idea of a strike-breaker:
4. slang. A term of abuse or depreciation applied to persons:
4.a. A mean, low, ‘scurvy’ fellow; a rascal, scoundrel. †occasionally applied to a woman.
a1592 Loue is such a proud scab, that he will neuer meddle with fooles nor children. R. Greene, Frier Bacon (1594)
This lasted until at least the late 19th / early 20th century
1899 You're three beastly scabs! R. Kipling, Stalky & Co.
The meaning in question first appears in the late 18th century and combines the moral and spiritual aspect found in the 16th century and applies it to the OP’s example
4.b. A worker who refuses to join a trade union, guild of fellow workers, etc.; in extended uses: a person who refuses to join a strike or who takes over the work of a striker; a blackleg; a strike-breaker.
1777 To the Public. Whereas the Master Cordwainers have gloried, that there has been a Demur amongst the Men's and Women's Men;—we have the Pleasure to inform them, that Matters are amicably settled... The Conflict would not been [sic] so sharp had not there been so many dirty Scabs; no Doubt but timely Notice will be taken of them. Bonner & Middleton's Bristol Journal 5 July
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. in A. Aspinall, Early Eng. Trade Unions (1949)
Scab isn’t a position, but a person who crosses a picket line when the union is striking, or, sometimes, a person who takes non-union work.
This article on the Boilermakers web page supports the negative view of the scab with a 1910 cartoon.
Merriam Webster defines this meaning of scab as:
3a: a contemptible person 3b (1): a worker who refuses to join a labor union (2): a union member who refuses to strike or returns to work before a strike has ended (3) : a worker who accepts employment or replaces a union worker during a strike (4) : one who works for less than union wages or on nonunion terms
As you say scab oldest meaning is that if an unpleasant person:
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.(Word Detective)
- 1594 [UK] Greene Frier Bacon and Frier Bungay C3: A companie of scabbes, the proudest of you all drawe your weapon if he can.
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. (Word Detective)
(also scabbie, scalie) a strike-breaker or anyone who stands out against a mass action; cite 2010 refers to non-union labour irrespective of strike action.
1777 [UK] Bonner and Middleton’s Bristol Journal 5 July n.p.: The Conflict [shoemakers’ strike] would not been so sharp had not there been so many dirty Scabs.
1806 in J.R. Commons A Documentary Hist. of Amer. Industrial Society III 74: I concluded at that time I would turn a scab [DA].
2010 [US] (con. 1973) C. Stella Johnny Porno 114: There were problems with a union delegate about using scabs.
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. (Word Detective)
(Green’s Dictionary of Slang)
