I saw “under pressure” in a description of a machine that uses a pressure lower than the ambient pressure to do its work, and I am pretty sure that is a mistranslation of the German »Unterdruck«, which literally translates to “underpressure” (note this is one word, whereas the translation used is two words). I was surprised to find “underpressure” as a possible translation of »Unterdruck« using LEO, which was confirmed by Wiktionary, but Merriam-Webster only forwards to me to “under pressure”.
Wiktionary describes its meaning as
insufficient pressure; a marked drop in pressure
and lists “overpressure” as an antonym.
This leads to my question of: What does “underpressure” mean exactly?
I have a feeling that it describes more of a non-intentional, out-of-spec low pressure (similar to how I understand “overpressure” as opposed to “high pressure”), whereas an intentional low pressure like you would expect in that machine would be called one of the following: