Definition of pop-in:
(n) :
(countable) A loud, sharp sound, as of a cork coming out of a bottle, especially when the contents are pressurized by fizziness.
(n) :
(uncountable, regional, Midwestern US, Canada, Inland Northern American, Midlands, Northwestern US, Western Pennsylvania, Northern England) An effervescent or fizzy drink, most frequently nonalcoholic; soda pop.
(n) :
(countable, regional, Midwestern US, Inland Northern American, Northwestern US, Canada, Western Pennsylvania) A bottle, can, or serving of effervescent or fizzy drink, most frequently nonalcoholic; a soda pop.
(n) :
A pop shot: a quick, possibly unaimed, shot with a firearm.
(n) :
(colloquial, in the phrase "a pop") A quantity dispensed; a portion; apiece.
(n) :
Something that stands out or is distinctive to the mind or senses.