drop in

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun One who arrives unannounced or without an appointment.
• noun surfing the act of dropping in (see verb below). Often hyphenated drop-in.
• verb idiomatic To arrive unannounced or with little or no warning; also, to visit without an appointment.
• verb surfing to paddle into and take off on a wave another surfer is already riding.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• verb visit informally and spontaneously