The Full form of BFF is Best Friends Forever. BFF is a term of endearment used to describe a very close friendship, typical of teenagers and young people. It is a form of writing digital affection in the 21st Century. BFF is often used by teenage girls as an adolescent expression to express camaraderie, but it is also sometimes used by girls in their 20’s and 30’s to humorously describe their close friends. This expression is used both in uppercase (BFF) or lowercase format (Bff) when typed into an instant message or email.