Anti-Fashion is an umbrella term for various styles of dress which are explicitly contrary to the fashion of the day. Anti-fashion styles may represent an attitude of indifference or may arise from political or practical goals which make fashion a secondary priority.
Anti-Fashion dates back to the 1850s in Victorian England, but these "anti-fashion" outfits were worn for functional reasons like riding a bicycle or swimming. In the 1950s, however, women would often choose to, instead of wearing the traditionally feminine-coded clothing, wear t-shirts, jeans, plaid shirts, and surplus military clothing, and this continued throughout history, leading to the births of both Punk and Grunge aesthetics. Popular designers in the Anti-Fashion world include Vivienne Westwood (the mother of Punk fashion herself), Issey Miyaki, Rei Kawakubo, Yohji Yamamoto, Ann Demeulemeester, Maison Martin Margiela and Raf Simons (the last three in particular were considered pioneers in the realm of Anti-Fashion in the 1990s).
Anti-Fashion visuals often carry an air of rebellion and a rejection of the status quo of the particular time period it comes from. Even in the era of "fast fashion" that we currently live in, there are people rejecting the current fashion norms by going in opposite directions (be it through thrifting, buying through independent clothing brands online, or some mixture of the two).
Anti-Fashion applies to many trends, but anything in the trend made avant-garde or misused can work. The feel of the trend without the effort usually involves some element of drugs or carelessness. Anti-Fashion is, more of then than not, done to completely go against established fashion trends until the fashion world, eventually, co-opts the fashions of the Anti-Fashion world and attempt to make it trendy and cool, which inevitably forces the Anti-Fashion world to evolve and change.