Not allowed to define "C" as name

I happily named some cells with one letter names: L, D, O, P … like that.

Because I like one letter variable names for ease of use when writing out formulas.

The system jibbed at ‘C’ because of something about it being a cell reference. I was in column C at the time.
So I played around a bit to see if that was the reason but it didn’t seem to be.
It seems of all the alphabet letters it didn’t like ‘C’. Why?

And what got the title of the question to do with the details of the question? Please adapt accordingly.

The dialog message is: “Invalid name. Reference to a cell, or a range of cells not allowed.”

C is the column reference notation if the Excel R1C1 formula syntax is activated (Tools → Options → Calc → Formula) hence can not be used as a range name; same for R the row reference notation. E.g. =R[-1]C means same column one row up, which in the Calc A1 formula syntax is =A1 if entered in cell A2.