To be suitable for.
It fits the purpose.
To conform to in size and shape.
If I lose a few kilos, the gorgeous wedding dress might fit me.
To be of the right size and shape
I wanted to borrow my little sister's jeans, but they didn't fit.
(with to) To make conform in size and shape.
I want to fit the drapes to the windows.
To be in agreement with.
These definitions fit most of the usage.
To adjust.
The regression program fit a line to the data.
To attach, especially when requiring exact positioning or sizing.
To equip or supply.
The chandler will fit us with provisions for a month.
To make ready.
I'm fitting the ship for a summer sail home.
To be seemly.
To be proper or becoming.
To be in harmony.
The paint, the fabrics, the rugs all fit.
Suitable, proper.
You have nothing to say about it. I'll do exactly as I see fit.
Adapted to a purpose or environment.
survival of the fittest
In good shape; physically well.
You don't have to be a good climber for Kilimanjaro, but you do have to be fit.
Sexually attractive; good-looking; fanciable.
I think the girl working in the office is fit.
Prepared; ready.
To suffer a fit.