(Cantonese) What's the right context in which to say, "三九唔識七"?
The phrase is used when you want to stress that some two people are complete strangers to each other. Examples:


我同佢三九唔識七，點敢叫佢幫忙呀？(I am a complete stranger to him. How can I bother him for help?)
阿仔，你同人三九唔識七，唔好咁易信佢呀！(My son, you know nothing about him. Don't trust him so easily.)


However, please note that the phrase is used essentially in spoken form but not in formal writings.