When and how did “scab” come to me someone who breaks a union picket line?
I always thought this was a rather insulting metaphor used by labor unions to talk about temp guys. The idea being the temps were, well . . . not permanent - but only there until the rift between union and management had healed, and then they disappeared. But I could be WAY off on this, it is just a guess.
“Solos Dios basta” . . . but a little pizza won’t hurt.
Here’s one view of its source: http://www.word-detective.com/041899.html#scab
My sixth grade English teacher was a liberal ex-nun. One day during a class discussion, somebody told a story about a piece of good fortune, and Gary Meunch reflexively called out “you lucky scab!” At the time (1972), “lucky scab” was a common term on our playgrounds, with no derogatory connotations.
Our teacher was absolutely aghast, and wanted to know where the phrase ccame from. Not being entymologists, we had no idea. She then told us about labor scabs and what terrible people they were. Not being laborers, we were unimpressed.
Totally OT, that same teacher introdcued us to the word “league,” in one of those horrid stories they make you read in sixth grade. A Western shoot-‘em-up contained the deathless line "I think ol’ Tom’s in league with the Clancy brothers." Teacher stopped (probably because her edition told her to) and asked us if we knew the new vocabulary word. Of course, we all responded. To be in league with someone is to be an enemy.
Teacher was aghast. (We horrified her quite a bit, as I recall. It was great fun.) No no no, she said. It’s the exact opposite. To be in league is to be a friend, an ally. How could you get it so wrong?
Now it was our turn to be aghast. No, that can’t possibly be, we said. Just look at it: the Cubs and the Mets are in the same league, and they are bitter rivals… 
Absolutely 100% guaranteed true, both of 'em.
“The dawn of a new era is felt and not measured.” Walter Lord