Initially I thought this was a joke (Julia doesn't seem to be popular enough to be one of the cornerstones of Jupyter, compared to Python and R), but indeed, Jupyter's documentation says it's true:
> The name Jupyter comes from the three programming languages the project originally supported: Julia (ju), Python (pyt) and R (r).
You'd think so, but Julia has been around a while now. Julia was one of the first non-python languages added back when it was still called ipython. I remember sitting in a room at the CfA with Fernando Perez and Steven Johnson and hacking up then original integration. Don't remember exactly when that was but more than a decade ago.