An abrupt decision is one you make suddenly. An abrupt halt is one the Road Runner comes to when reaching an abrupt drop-off—you'd hear brakes squealing.
Because being abrupt with someone is often a way of showing them that you are angry, you often apologize for the appearance of it. "I'm sorry I was abrupt. I was rushing to get to class." The root of abrupt is Latin rumpere "to break," which also gives us rupture "break" and interrupt "break into."