John Legend and Chrissy Teigen were kind of Ricky and Lucy Tuesday night, when she got in the act and then jacked his performance with the help of a nip slip.

John was smooth as silk Tuesday night while singing "Slow Dance." That's when he brought Chrissy up for some slow dancing. It all looked fantastic ... and then the boob -- let's get real, even more fantastic!

Chrissy owned it, and the show went on.

#RelationshipGoals