19's Fear

Listen up, smarty pants! The joke is that 420 is a number associated with marijuana. Nineteen is "afraid" because if you add 400 to it, you get 419, which is close to 420, and that makes it sound like 19 is about to get exposed to the devils lettuce, which would make him paranoid. The joke relies on the listener knowing 420 is a reference to weed, something I know you wouldn't know about!