Meaning

noun a vigorous blow

noun a sudden very loud noise

noun a border of hair that is cut short and hangs across the forehead

originFrom bang (“to cut [hair], dock [an animal's tail]”), probably from the sudden manner in which docked hair appears to burst out explosively when it's cut (i.e. "with a bang"). Ultimately from bang (“to strike noisily, explode”).

Etymology from Wiktionary, CC BY-SA 3.0