Go sleep near a piano is a saying to tell someone to go ahead, although they will end up doing the opposite of what they wanted to do. This is because if you sleep near a piano, ur head will fall on the keys making a big noise and keeping you awake. Therefor doing the opposite of sleeping.
Brother: wassup my g, imma smoke weed.
Me: go sleep near a piano.
(Brother goes to smoke weed but then changes his mind and decides to help people on the street get off drugs.)