In the 1980s I was in the IEEE committee that wrote the standard that ultimately became known as POSIX. The committee set itself the task of standardizing interface specs for a Unix-like system, but had no short name for its work. When the first part of the specification was ready, someone gave it the name "IEEEIX", with a subtitle that included "Portable Operating System" — perhaps "Specifications for a Portable Operating System".
It seemed to me that nobody would ever say "IEEEIX", since the pronunciation would sound like a shriek of terror; rather, everyone would call it "Unix". That would have boosted AT&T, the GNU Project's rival, an outcome I did not want. So I looked for another name, but nothing natural suggested itself to me.
So I put the initials of "Portable Operating System" together with the same suffix "ix", and came up with "POSIX". It sounded good and I saw no reason not to use it, so I suggested it. Although it was just barely in time, the committee adopted it.
I think the administrators of the committee were as relieved as I was to give the standard a pronounceable name.
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Link: The origin of the name POSIX.