It's in the script.
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"You boys know me. I ain't a red boy one bit! Here I'm carryin' a shrapnel that big I picked up in the war. And maybe I don't know it when it rains! Don't tell me red! You know what we are? The black and blue boys! We been kicked around so long we're black and blue from head to toes. But I guess anyone who says straight out he don't like it, he's a red boy to the leaders of the union. What's this crap about goin' home to hot suppers? I'm asking to your faces how manys got hot suppers to go home to? Anyone who's sure of his next meal, raise your hand! A certain gent sitting behind me can raise them both. But not in front here! And that's why we're talking strike - to get a living wage!"
Well, I found the text, but I don't think the line you quoted above, by itself is very significant. I believe the speaker is hinting at the fact he's not the brightest of the bunch, but that he served his country, carries shrapnel in his body from fighting for it, and that he's there to speak up on behalf earning a good living.
Waiting for Lefty
He's refering to the pain the shrapnel causes him when it rains. He knows what pain feels like.
Texting the script.