When you hear pi-r-squared you probably think of a circle but historians have uncovered notes to suggest that it came from bakers making pies which after going through the baking process some parts changed from being circular to square.
One baker Joseph Flapper from Plate Row in London was known to make his family pies with a specific radius of four inches, but how deep they were is unknown as he was more concerned with the top of the pies. He called the top of his pies an "arr".
Knowing how the shape of the arr changed from circular to square during the baking process due to his recipe Mr Flapper was often heard saying "Pie arr squared" with pride as he took his pies out of the oven. As it was almost square Mr Flapper would then measure the length of a side of the arr and he was happy if the side measured approximately seven inches as this convinced him it was baked to perfection.
Somehow 7/4 was used as an approximation factor for transformation of the size of pies by Flapper's Bakery but wasn't very practical. It was only when mathematicians started working on circles, and having heard of Flapper's "Pie arr squared" that it became part of the formula for the area of a circle. Mathematicians decided that this would be the ideal formula if the special constant needed was called something phonetically equivalent to pie, hence pi.
So next time you quote the formula for the area of a circle, this expression was used by Joseph Flapper long before it became a standard formula in maths.