'Ey Hed!' is a running gag related to a reaction usually made by Hellucard aka Dom . The gag is usually used when Dom sees Edd. It's actually supposed to be "Hey, Edd!" but it sounds more like "Ey Hed!".
Dom sees a bunch of Edd clones including one naked. He repeats saying "'Ey, Hed!" several times.
Edd mistakes the big wave for Dom's waving hand while he says "'Ey, Hed!"
When the trio go to the diner, Dom says "'Ey, Hed!" before they go in. Then after the waitress gets angry, Edd hears "Hey, Edd!", shortly before Dom is killed by being thrown through a glass door by Future Edd.