grammar - Is the sentence "What to do?" very old fashioned idiomatic English, or is it simply ungrammatical?

On a foreign-language-learning forum there is a question that's given the English translation "What to do?"

My impression is that that's an example of very old fashioned English. Maybe something I'd expect in Shakespeare or at least a snooty upper class character in an old black-and-white movie.

Or am I mistaken and it's just a simple bad translation into an ungrammatical sentence lacking a main verb?

References:

At Mari-Lou's suggestion, here are some of the comments against. There are also comments for, which I'm not listing. You can scan through the thread if interested:

• Not at all common without a subject. More common: What am I doing?


• It may be heard in the UK, but it is not common. In context, it would probably be understood. But from a native speaker, it would sound archaic and peculiar.


• Native speakers in the US never say this!


• I have never heard anyone say What to do? in English


• "What to do?" Makes no sense as an English sentence.


• This is not a widely accepted way to express oneself in English.



• In English " To do what? or Do what! would be normal but not "What to do?" as a question is not correct in my opinion.


• I don't agree with Duolingo's translation on this one. In Spanish you can ask "Que hacer?", but in English "What to do?" is not a grammatically correct question.


• "What to do?" sounds like a mistake a Spanish-speaking person would make at an early stage of learning English.


• I certainly recognise what you say, but it doesn't sound like English as I speak it.


• Does any english speaker actually say "what to do?" I definitely don't