drop in (plural drop ins)
- One who arrives unannounced or without an appointment.
- The office was efficient, but not well equipped to handle drop-ins.
- (surfing) the act of dropping in (see verb below). Often hyphenated drop-in.
- 2005: Drop-ins can and do happen by accident, as well as through frustration and confusion in a crowded lineup. — surfline.com Surfology 
- (idiomatic) To arrive unannounced or with little or no warning; also, to visit without an appointment.
- I was in the garden covered with mud when my grandmother dropped in for a visit.
- (surfing) to paddle into and take off on a wave another surfer is already riding.
- Most beginners are blissfully unaware that dropping in on someone is a cardinal sin — Neal Miyake, The Unwritten Rules of Surfing