On the nurses’ board, under “diet”, it said NPO; and if you asked if you could have some juice or whatever, nurses would tell you no, you were NPO — and then maybe they’d explain that meant ‘nothing by mouth’.
Why should NPO be an abbreviation of Nothing By Mouth? If they’d once learned why, they’d forgotten, and now it was just medical jargon with this meaning, and many of them no longer realized that ordinary people might be baffled by the claim that NPO was an abbreviation for Nothing By Mouth (for which the alphabetic abbreviation would be NBM).
But it is an abbreviation. Of Latin Nil Per Os — more exactly, Nil / Nihil Per Ōs, where nil is a contraction of nihil ‘nothing’ (as in English nihilism) and ōs (the object of the preposition per) is the acc sg of the 3rd-declension ‘mouth’ noun with nom sg ōs and gen sg ōris (as in English oral).
But in any case, users of jargon — expressions associated with particular occupations or activities — are very often not aware of its in-group status and aren’t prepared to explain it to outsiders; it’s just the way you talk in this context.
