ahurmazda pointed out that you're thinking of 'Jupyter Python R'. And this is a reference, not an 'equals'.

The name is also in homage to Galileo's notebooks recording the discovery of the moons of Jupiter (the four we now call the 'Galilean moons'), and also to a bar called Jupiter in Berkeley, which the core team has visited quite often. The last one's more like a funny coincidence, though.