It is similar to giving a high five except instead of putting two hands together, one finger is put across another person's two fingers, forming the mathematical pi symbol.
You did so well on th math test, the teacher gave you a high pi.
Very much like a high five, except 3.14159... fingers are thrown instead of the usual five.
Hardy: "The number 1729 seems to me a rather dull one..."
Ramanujan: "No, it is a very interesting number; it is the smallest number expressible as the sum of two cubes in two different ways."
Hardy: "Oh, snap! High Pi, son! Damn!"