P's and Q's - P's = paper money. Q's = drugs. So when someone tells you to

P's = paper money. Q's = drugs. So when someone tells you to watch your P's and Q's, they're warning you not to get caught with both paper and drugs.

Watch your P's and Q's, or I'll get arrested.

Don't mix paper and drugs, or I'll go to jail.

P's and Q's, don't let the cops catch you with both.