Literally "to play the lute to a cow"; used to describe talking to someone who cannot understand your point, or aiming high-level talk at the wrong audience.

to talk nonsense or act recklessly; literally "to play the piano at random"

(xiehouyu) To find joy or amusement in the midst of adversity or suffering; literally "playing a stringed instrument under a coptis tree," since the tree is famously bitter while the music is a pleasure.