"Ya nan" is commonly used as an insult towards people who you dislike or envy. Ya is and abbreviation of the word your. The use of their Nan in the insult shows you are willing to 'dis' their Family!
The phrase "suk ya nan" is a highly intelligent phrase used by the most political people on our beutiful world. It means to pleasure your grand mother by sucking parts of her body with great delight.