The question of whether or not the term "fat" is a derogatory word has a number of answers. Some people say that it is just an adjective and attempt to reclaim the term and strip it of its loaded meaning. Other people feel that using this term to describe someone is offensive, because of the cultural ideas which surrounding the term, and they prefer to use words like “overweight.” Generally speaking, it's best to think carefully before using this word to describe someone, unless he or she has personally used the word, because it is a very loaded term.
In the literal sense, fat is just a chemical compound, but the term has come to be used as an adjective to describe people who have large deposits of this compound under their skin. In some cultures, large deposits of fat are deemed physically unattractive, and therefore the adjective has become an epithet, because the assumption is that looking a certain way is not desirable. Calling someone fat is supposed to be insulting, even when it is an accurate description of his or her body, because the name-caller assumes that the person is uncomfortable or perhaps unhappy with the way he or she looks.
In the late 20th century, many activists and radical groups attempted to reclaim words which had historically been used as insults. The idea was that rather than allowing people to use language as a tool for verbal violence, people could embrace such terms, thumbing their noses at critics. Some feminists, for example, embraced an epithet which is used to describe female dogs, while some people with non-heteronormative sexualities started referring to themselves as “queer.” Activists who focus on sizeism suggest doing the same thing with “fat.”
Many overweight activists refer to themselves as fat, and they may reference this in the materials they distribute. Some make a distinction between “fat” and “overweight,” arguing that “overweight” implies that there is an acceptable or normal weight beyond which people should not go. By using the word frequently, they hope to reduce the sting of term as an insult, and they invite people of all sizes to use it as an adjective to describe people who are overweight. However, other people feel that even though a valid attempt to put this word back into its linguistic place is being made, the term should not be used without careful thought.
Some people believe that the term should only be used by people who make it clear that they are comfortable with it and that other people should avoid using this word to avoid causing offense if they're not sure how a person feels. Others feel that all people who are friendly to the idea of body acceptance should be permitted to describe people as “fat,” and to discuss the issue of sizeism and body positive activism with people who question the use of this word.