Definition of "foo"

(historical, obsolete) Alternative form of fu: an administrative subdivision of imperial China; the capital of such divisions.

(programming) A metasyntactic variable used to represent an unspecified entity. If part of a series of such entities, it is often the first in the series, and followed immediately by bar.  examples

(fandom slang) Alternative letter-case form of Foo (“placeholder god”)

Expression of disappointment or disgust.  examples

(slang) Pronunciation spelling of fool.  quotations