Formal:
* Unbecoming
* Unseemly
* Improper
* Unsuitable
* Unfit
* Out of place
* Inexpedient
* Undesirable
* Inapplicable
Informal:
* Out of line
* Uncalled for
* Off-color
* Tacky
* Crass
* Uncool
* Unprofessional
* Disrespectful
* Offensive
More Specific:
* Indecent (for behavior that is sexually suggestive or offensive)
* Unorthodox (for something that deviates from accepted norms or practices)
* Unconventional (for something that is unusual or non-traditional)
* Unconscionable (for something that is morally wrong or unacceptable)
The best synonym will depend on the specific situation and the tone you want to convey.