Foo definitions

(computing) 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.

Suppose we have two objects, foo and bar.

noun
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Expression of disappointment or disgust.

Oh foo - the cake burnt!

interjection
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A popular name for a temporary file, function or variable, or an example of same in documentation. Often used with "bar" to create "foobar," which is a variation of FUBAR. FUBAR came out of World War II and means "F***ed Up Beyond All Recognition."
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(slang) Fool, foolish person.
noun
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(slang) Alternative spelling of foo (short form of fool)
noun
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Origin of foo

Circa 1935 as nonsense word, circa 1960 in programming sense.