Antonyms for the word "being proper for" would include phrases like "inappropriate for," "unsuitable for," or "inapt for." Using any of these antonyms would convey the exact opposite of being proper for something. For example, if a person were to wear ripped jeans to a job interview, this would be inappropriate for the occasion and would not be seen as being proper for the interview. Similarly, if an investigation were conducted by an unqualified individual, the findings could be considered unsuitable for the situation. So, depending on the context and situation, there are a variety of antonyms that can be used to describe something that is not appropriate or proper.