The undercut is a hairstyle that was fashionable during the Edwardian era, 1920s, 1930s, 1940s, and 2010s predominantly among men. Typically, the hair on the top of the head is long and parted on either the side or center, while the back and sides are buzzed very short. It is closely related to the curtained hair of the mid and late 1990s, although wearers of undercuts during the 2010s tend to slick back the bangs away from the face.
Historically, the undercut has been associated with poverty and inability to afford a barber competent enough to blend in the sides, as on a regular haircut. From the turn of the 20th century until the 1920s, the undercut was popular among young working class men, especially members of street gangs. In interwar Glasgow, Neds, the precursors to the Teddy Boys, favoured a haircut that was long on top and cropped at the back and sides. Despite the fire risk, lots of paraffin wax was used to keep the hair in place. Other gangs who favored this haircut were the Scuttlers of Manchester, and the Peaky Blinders of Birmingham, because longer hair put the wearer at a disadvantage in a street fight.
During the jazz age of the 1920s and 1930s, hairstyles of this type were considered mainstream fashion. In Nazi Germany, a version of this haircut which was long on top but shaved at the back and sides was popular among Wehrmacht officers. – Source: Wikipedia
Example of this picture below is Brad Pitt with slick back – undercut hair style