drop in (plural drop ins)
- One who arrives casually and unannounced, 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 
drop in (third-person singular simple present drops in, present participle dropping in, simple past and past participle dropped in)
- (idiomatic) To arrive casually and unannounced, 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.
1898, Winston Churchill, chapter 5, in The Celebrity:
We made an odd party before the arrival of the Ten, particularly when the Celebrity dropped in for lunch or dinner.
- (surfing) To paddle into and take off on a wave another surfer is already riding.