Pictogram of the spine (backbone); two 二 characters are the shoulder-blades or ribs. 人 character is like an arrow pointing up to show the spine going up the back. 月 character is the two legs and two vertebrae. It’s a stylized picture of the human bones from the back.
二 人 二 月 s P JI SPINE E Gee! J.oInts I.n SPiNE!