drop in

verb

Used other than figuratively or idiomatically: see drop, in.
“The keys were dropped in the fishbowl.”
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.”
“We made an odd party before the arrival of the Ten, particularly when the Celebrity dropped in for lunch or dinner.”
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.”
To fall in; to cave in.
“⁠The cheeks drop in; the body bows; Man dies: nor is there hope in dust: […]”
To install components, particularly car engines.
“We dropped in all-new parts in the sports car.”