In surfing, the right-of-way violation of taking off on a wave someone is already riding. In skateboarding, dropping into a ramp from the coping.
The term carries two meanings worth keeping apart. In surfing, dropping in is a rule violation: you take off ahead of someone closer to the peak who is already riding — dangerous, since boards with sharp fins collide, and the classic source of line-up conflict. If it happens by accident, kick out immediately and apologise; that settles it. In skateboarding, by contrast, a drop-in is a technique: set the tail on the coping, put your front foot over the front truck and commit your weight forward. Hesitation is the actual mistake — lean back and the board slides out from under you.