He has thus "cut you off" from your rightful place in traffic, since inevitably the cars behind him will follow his example and you are hopelessly stuck in the slower lane.
***This is not to be confused with purely idiotic driving. There are many situations where slower moving traffic moves into your lane ahead of you. This may cause you to be cut off by those behind you, but the act of getting in front of someone who is going faster than you is not so malicious as it is plain idiocy.
Please, use this term correctly. There is a special place in hell for people who intentionally cut off other people when clearly that person wanted to use the next lane as well.