What, exactly, is "dropping in"? When you are out surfing, there are rules and conventions. The most important is the rule of priority. Priority means that the surfer who takes off closest to the peak owns the wave.

Dropping in is getting onto the wave in front of the surfer with priority. Very bad manners, and potentially dangerous too. Especially if you are a body boarder. Body boarders are sometimes derogatorily called "speed bumps", and that is what they will most likely become if they drop in on a surfer.