not appropriate for (something)
When you say that something is "not appropriate", it means that it's bad. But the word "bad" means that something isn't good in any situation, while "inappropriate" means "bad" for a specific situation. You tell what situation you're talking about with "for ___". For example:
Did you see what she's wearing? That's really not appropriate for work.
You can also follow "appropriate" with "to (do)":
That's a good song, but it's not appropriate to play at a wedding.
The word "appropriate" is intellectual- and adult-sounding.