John Legend Roasted After Covering Nate Dogg's Hook in '21 Questions'

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John Legend got onstage with 50 Cent to do his best version of Nate Dogg -- and while he certainly dished his own twist ... not everybody's feeling it, and they're letting him know too.

The singer celebrated his 45th birthday Thursday in NYC -- where he and Chrissy Teigen hired Fitty to perform, with the rapper doing his classic "21 Questions" live in front of a crowd. At first, it was just 50 Cent handling the hook and verses ... but JL eventually jumped in.

Very impromptu, John grabbed the mic from 50 and started belting out the chorus ... singing, "Girrrrl, you seem to love me now // Would you love me if I was down and out."

Looks like John kept singing while 50 continued to rap -- and in the moment, everyone seemed to receive it pretty well ... cheering on the bday boy and giving him props.

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