[nkurz]

It apparently was chosen because it's the starting sound of "turn": Hartl chose tau to represent 2pi because it nicely ties in with the Greek word “tornos,” meaning “turn,” and “looks like a pi with one leg instead of two.”

https://blogs.scientificamerican.com/observations/the-tao-of...

There was an earlier effort that used a new "two pi" symbol consisting of a "π" with an extra leg in the middle: https://www.math.utah.edu/~palais/pi.pdf.

[krastanov]

Funny coincidence: π with an extra leg is the Cyrillic cursive letter for the sound t.