What does Chai Shen mean?

1) If it is a friendly joke between friends, it should be.

You are too thin.

2) if it is not between friends, but with some malice, it means

You are so pustule, you are a big loser.

Tongfagai

Waste wood is a common slang in Cantonese, and it is also a slang in Hong Kong. Refers to useless people, or waste, meaning "waste wood" and "pustule".