Define stack: a usually neat pile: a group of things that are put one on top of the other — stack in a sentence. Stack definition, a more or less orderly pile or heap: a precariously balanced stack of books; Meaning "set of shelves on which books are set out" is from Haystack · Stack up · Stack the cards · Air stack. This Internet Slang page is designed to explain what the meaning of STACK is. The slang word / acronym / abbreviation STACK means . Internet Slang. A list of.