Someone who has no awareness of anyone else on the road around them. Or, someone who doesn't care about anyone else on the road: the "I'm doing this and that's that" type of driver. For example, changing lanes without indicating or checking blind spot and completely cutting you up or almost driving into you, or for example, on a dual carriageway or even a motorway, tailgating you, flashing at you and beeping at you while you're trying to make sure you've cleared enough room to change lanes, so that you won't cut up someone else in another lane. DEAR LORD.
Or, people who CANNOT use roundabouts correctly.
I know that people have their off moments or off days, but I have seen the same drivers so many times doing exactly the same thing, if not to me, to someone else. If someone has an off moment they generally raise their hand in apology or something like that.