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It might mean "reinforce this argument/provide evidence for this", per https://constantrenewal.com/steel-man

From the context, it might also mean "rebut this as strongly as possible", "criticize this" or "defuse this argument".

It's unfamiliar wording to me as well. Does anyone have a rough etymology of "steelman" as a verb?

I think steelman is derived as the opposite of strawman. Taken literally, something made from straw is weak, and something made from steel is strong.

A strawman is an imaginary opponent who is easy to defeat. Like a scarecrow (a human-like figure stuffed with straw, meant to scare off birds from crops) which would be easy to defeat because it doesn't fight back when attacked.

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