As others have said it means "Highest Paid Person's Opinion" but as a little extra.
It is usually used when a company has no actual data to work on and therefore goes with the "obvious" as defined by "the boss" (Hippo).
Not that it is necessarily true. There could be lots of data that some people are not aware of and that only senior people are privy to, but if that is not shared with the team developing the software then that is in itself not a great sign.
Nah, HIPPO works perfectly well in "data-driven" companies: the highest paid person proclaims what the data is supposed to tell, and then lots of data "science" happens to make the data show exactly that.
High Importance Person Pushing Own (desires??)?