1. Informal.. Also, dropper-in.
a person who or thing that pays an unexpected or uninvited visit: a feeder for squirrels, raccoons, and other drop-ins.
a social gathering at which
the guests are not expected to stay long: Be sure to stop by our house for a glass of eggnog at our Christmas drop-in.
provided for short-term patronage: a drop-in shelter for the homeless.
requiring only insertion to be ready for use: a drop-in film cartridge.
1810–20; noun, adj. use of verb phrase drop in