Come from the English phrase "poor guy".
Back when England was the ruler of Hong Kong, the English officials felt pity for the beggars and said poor guy, but the Hong Kong people doesn't know English and said it as pok gai. Then later as the language evolved, pok gai turned into a negative term because pok sounded like the Chinese word "trip" and pok sounded like the Chinese word "street". Now people in Hong Kong say this phrase pok gai (aka PK) which can be used as a noun and verb.
The noun form - a person who deserved to trip and die.
The verb form - trip and die
Noun form - You god damn pok gai, watch where you are going.