A trope technique originating in tv professional wrestling.
Normally, when the wrestlers strike each other, they overreact to show the audience that it was a horribly strong blow. Remember doing fake wrestling moves as a kid and "hitting" your head on something where your hand is between your coconut and the hard object that would certainly make your brains leak out if you actually made impact, but you made your hand hit the object with a loud slapping sound and let your head softly hit your hand, then "selling" the blow by rolling around in fake terrible pain and maybe some fake blood? Yeah, it's like that run-on sentence.
No-selling is the opposite. When a nameless meat grinder wrestler hit Hulk Hogan with a closed fist, he would just stand there and look bemused. This is the "no-sell", where a landed punch hits the hero wrestler but they don't "sell" it to the audience, making it appear either that the other wrestler is weak or that the hero has amazing power and strength.
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