A Coulthard is someone who is generally very quiet and relaxed, but that has sporradic bursts of energy, at which time they will usually punch someobody, throw something, or maybe even kill those around them. Once said act has been commited by a person, all friends in the vicinity must immediately yell "Coulthard!!"
It also is used when reffering to someobody that is acting like they are cool. Coincidentily, if you put thard after anything that rhymes with "COOL" it sounds awesome.