Meaning retard, derives from the name of a 'short bus' company called Nebard's. Calling someone a nebard suggests one rides said buses and is therefore a complete fucking retard (as short buses are the common method of transport to and from school/college for mentally disabled students).
To be a retard - act retarded or stupid - yes it is taken from a bus company - this company is located in wakefield and me and my friends (people may contest this) were prob the first people to start using the name nebard back in early 2000.