preposition
A preposition is a word that shows the relationship between a noun (or pronoun) and other words in a sentence. In this case, 'ad' means 'to' or 'towards'.
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The project deadline has been postponed ad kalendas graecas.
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We'll probably finish this project ad kalendas graecas.
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I'll believe it when it's done ad kalendas graecas.
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Waiting for him to change his mind is like waiting for ad kalendas graecas.