General:
* Improper: This is a direct antonym of "proper" and is a good general term.
* Inappropriate: This implies something is unsuitable for a particular situation or context.
* Unfitting: This emphasizes that something is not a good match or is unsuitable.
* Unseemly: This suggests something is not appropriate or socially acceptable.
* Unorthodox: This indicates something is not conforming to accepted standards or practices.
Specific to behavior:
* Unruly: This describes behavior that is difficult to control or manage.
* Disorderly: This suggests a lack of order or organization.
* Misbehaving: This indicates that someone is acting in a way that is against rules or expectations.
Specific to things:
* Faulty: This describes something that is not functioning properly.
* Defective: Similar to faulty, but implies a flaw in the design or manufacturing.
* Incorrect: This indicates something is wrong or inaccurate.
* Inappropriate: This is a good general term for something that is not suitable for its intended purpose.
To choose the best word, consider the specific situation and what nuance you want to convey.