The shell bone script of character Fit is
The top is upper lip, the bottom is mouth, here is lower lip. The lips close together completely to cover the mouth, which means Fit.
The Dictionary of Pictography explains that the character means kissing, I don't agree. The two lips are from the same person, not two different person.
Some people believe that the top is a lid, the bottom is a pan, the two fit each other.
The bronze script of the character is