It's alluding to the nabla symbol (https://en.wikipedia.org/wiki/Nabla_symbol https://en.wikipedia.org/wiki/Nabla_symbol) used in mathematics, and the similarit

It's alluding to the nabla symbol (https://en.wikipedia.org/wiki/Nabla_symbol https://en.wikipedia.org/wiki/Nabla_symbol) used in mathematics, and the similarity in sound between the two does not go unnoticed by many students of calculus.