How have "in + [adjective]" type expressions, such as "in general", "in common" and "in private", come into use? I'm puzzled by these expressions, because I understand that a preposition is usually followed by a noun phrase. Is it the case that there used to be a noun after the adjective, for example "in general cases", and later it was dropped?
The examples that you give can all be extended thus:
- in general terms
- in common with each other
- in private circumstances
The examples offered by Edwin Ashforth in his comment can also be easily handled in this way. Of these, "in vain" probably requires explanation: it appears to have come from French "dans une vaine tentative," literally "in a vain attempt." (The Latin for "empty" or "without substance" is "vanus.")