It’s not a real sign. It’s a digitally-altered image created as an internet meme to produce a visual ‘rebus puzzle’. There are other variations too, like these, using the same template:
There are loads of different interpretations floating around, many of which are totally overthought. I believe the sign with the knee is meant to phonetically become “need to live”, the one with the rib-cage (bone) is meant to become “born to live”, and the one with the chair (seat) is meant to become “I foresee it” when said slowly. Nothing more than that, and part of the ‘joke’ (if you can call it that) is that the messages are mundane, rather than the speculated anticipation that they’re going to be something very rude.