For clothing:
* Loose
* Baggy
* Tight (if "fit" means too tight)
* Uncomfortable
For health/physical condition:
* Unhealthy
* Out of shape
* Weak
For size/shape:
* Mismatched
* Inappropriate
* Improper
For mental state:
* Unprepared
* Inadequate
* Unsuitable
For a puzzle piece:
* Incorrect
* Wrong
For a tool/equipment:
* Broken
* Damaged
* Faulty
To give you the most accurate opposite, please provide more context about how you're using the word "fit."