An exclamation used in response to seeing or hearing someone else being made fun of, or seeing someone fall down or make a fool of themselves somehow. Forcefully exclaiming "Deyaaa!" in a very high-pitched voice adds to the humiliation of whomever is being made fun of. The more a's added to the end of the word, the more intense the insult/crack/dis or humilation was. If you are ever being made fun of and are unfortunate enough to hear "Deyaaa", it will feel like it is piercing your soul.
Jerome: "I heard his daddy gots dickdo disease... His stomach hang out futher than his dick do."