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1. adverb Indirectly, as opposed to from the original source. But I heard thirdhand, so who knows if that information is actually true.
2. adjective Describing information that has been learned indirectly, as opposed to from the original source. I can't trust this thirdhand gossip! Who was in the meeting and can tell us what really happened?
3. adjective Describing an item that one has obtained after two previous owners. Try to pretend that this isn't a thirdhand dress and gush about how good it looks on me!