What part of speech is honesty in the sentence The best policy honesty?

In the sentence "The best policy honesty," honesty is acting as a noun.

Here's why:

* It's the subject of the sentence: The sentence is about honesty, not about a policy.

* It's not modifying another word: There's no adjective or adverb function for "honesty" in this sentence.

While the sentence is grammatically incomplete (it needs a verb), "honesty" is clearly intended to be the noun representing the subject of the sentence.