I'm only seeing both deadweight (one word, no hyphen) and dead weight (two words, no hyphen) being used as nouns. I'm not finding it anywhere with a hyphen. I don't think you can use it as an adjective in any case.
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I would have to agree that deadweight is not an adjective and many other adjectives could be used instead in your sentence. If you like the expression, try rewording your sentence. When I lifted her body up off the floor, it was like dead weight.
I came upon this expression while doing transcription and that caused me to research this. The sentence was from a Judge in reference