The full form of FWB is friends with benefits
The acronym “FWB” stands for “friends with benefits”.

This generally means a friend in which you have a sexual relationship with, but do not label themselves in a relationship. It is not uncommon to see “FWB” already be involved with other people, while still maintaining their “FWB” status. “FWB” often go on dates, as friends, meaning they’re just hanging out; they may use pet names; they talk all the time and are extremely close, and they have a sexual relationship in addition to their friendship. However, they do not claim a personal, monogamous relationship status.

This internet slang term can also describe two people who act as if they’re in a relationship, such as going out together, cuddling, holding hands, but never kissing or having sex. But they still have most of the benefits that come with being in a relationship. They are “FWB”. It doesn’t have to be sexual to make this acronym work