I read once that the mathematical use of π originally was in reference to circumference (perimeter, hence Greek P), not the ratio of circumference to diameter, so the shift of the symbol's interpretation to how we use it now is an example of mathematical metonymy

Also, Greek π was adapted from a Phoenician glyph for /p/ which, prior to acrophonia, was a logogram for 'mouth' (i.e. piehole apparently)