"hair cut straight across so as to form a fringe over the forehead," 1878 (in singular, bang), American English, attested from 1832 of horses (bang-tail), perhaps from notion of abruptness (as in bang off "immediately, without delay," though this expression is attested only from 1886). See bang.

1580s, "sudden, unceremonious, without notice," a figurative use from Latin abruptus "broken off," also "precipitous, steep" (as a cliff), also "disconnected," past participle of abrumpere "break off," from ab "off, away from" (see ab-) + rumpere "to break," from a nasalized form of the PIE root *runp- "to snatch" (see corrupt (adj.)). The literal sense "broken off or appearing as if broken off" is from c. 1600 in English. Of writing, "having sudden transitions, lacking continuity," 1630s. Related: Abruptly; abruptness.