Harry Styles paid tribute to 1D, as in ... DEEZ NUTS ... in Las Vegas.
One Direction won the Billboard Music Award for Top Duo/Group, and Harry celebrated by slyly grabbing a hold -- playfully and gently, we hope -- of bandmate Niall Horan's balls.
Watch ... Niall plays it off pretty good.
Not sure Harry even asked him to cough first. So rude.