"Bangs," the hair style, comes from the same roots as "bang," the sound of a gun, a slamming door, or countless other abrupt noises. The word "bang" first appeared in written English in the 16th century, but is thought to have been known in the dialects of Northern England long before that date. "Bang" comes from an Old Norse word "banga" meaning "to hammer," and is a linguistic relic of the Viking invasions of England beginning in the eighth century. "Bang" at first meant "to strike violently," but gradually the word came to be used for any sudden or violent movement, especially one which caused a loud noise. One of the earliest written examples of this expanded sense of "bang" refers to slamming a door, an apparently universal human action which may yet prove to be as great an instrument of self- expression as the typewriter. Aside from doors, nearly anything could go "bang," from guns to pianos, and "bang" also came to mean fight or beat up.

"Bang" continued to evolve, and by the 19th century was used to convey suddenness or finality, which brings us at last from Old Norse hammers to modern haircuts. "Bangs" are so-called because they are created by cutting the hair "bang- off," abruptly and straight across the forehead. And finally, at the risk of offending our bang-coiffed readers, I must tell you that "bangs" as a young lady's hairstyle almost certainly originated with the practice of cutting horses' tails straight across, a style known to this day as a "bang-tail."
answer from worddetective.com