a cantonese slang, to die. Literal meaning is to fall down out on the street and die. "pook" is the sound of the head hitting the pavement. "Gai" is the street.
It is used when someone is angry at someone or something and wishes the person dead. Or just used when one is angry.
(i am adding this because none of the other postings had the literal translation of the term)
Damn...that pok guy just ran a red light and almost hit me!