The most common word was "Kiffle." DarkViperAU noted that the word had been used over 500 times.
Those who have played GTA 5 probably know that there is a religious cult of Epsilon in action. And it is epsilons who always say "Kifflom" when parting and greeting. Therefore, when Michael wears the robe of the cult, passers-by greet him that way.
Interestingly, they started talking about Kifflom back in GTA: San Andreas. He is the deity of the cult they worship. San Andreas states that Kifflom last came to Earth in a chicken with broad wings and four legs.