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).
> The name Jupyter comes from the three programming languages the project originally supported: Julia (ju), Python (pyt) and R (r).
https://docs.jupyter.org/en/latest/what_is_jupyter.html