The symbol that we are used to seeing today has not always been used to denote Pi. One of its first uses was as late as 1647 when Oughtred stated that the ratio of the diameter of a circle to its circumference was:
d / p
In 1697 David Gregory also used the symbol in his notation. He stated that the ratio of a circumference of a circle to its radius was denoted by:
p / r
It wasn't until 1706 that the symbol for Pi that we use in the current day was first defined. William Jones was the first person to define the number as:
3.14159... = p
In 1737 Euler used the symbol for the same purpose and it quickly came into widespread use.
Tenuous Link: Piperidine
http://www.ansci.cornell.edu/plants/toxicagents/piperidi.html#intro