foo

Short definition from Wiktionary

Noun

1. 1
   Alternative form of fu: an administrative subdivision of imperial China; the capital of such divisions.
2. 2
   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.
3. 3
   Alternative letter-case form of Foo (“placeholder god”).
4. 4
   Pronunciation spelling of fool.

Interjection

1. 1
   Expression of disappointment or disgust.

Proper noun

1. 1
   A mock deity of early science fiction fandom; a fannish ghod.