Slut shaming, sometimes called sexual bullying, is a concept used to describe the act of making any person feel guilty, inferior or undesirable for certain sexual behaviors or desires that deviate from traditional or orthodox gender expectations, or that which may be considered to be contrary to natural, societal, cultural, or religious law.
When done with consent, slut shaming can be considered a form of humiliation play.
Some examples of behaviors over which women are said to be "slut-shamed" (though men may also be slut shamed) include: violating accepted dress codes by dressing in sexually provocative ways, requesting access to birth control, having premarital or casual sex, or being raped or sexually assaulted. While slut shaming has most often historically been directed at women, anyone of any sex, gender, or sexual orientation can be a victim of slut shaming, and anyone can be a perpetrator of slut shaming. 
Non-consensual slut shaming is to be avoided as it can induce anxiety and sexual repression in others, or damage their self-esteem. If you are concerned someone is acting without morals or ethics, consider asking them open-ended discovery questions designed to reach understanding rather than to voice hurtful observations.