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Thick as thieves

Posted by Michael on September 04, 2000

Does anyone know the actual meaning and/or origin of the phrase "thick as thieves"? I've been told that it means "numerous" as in "The mosquitos were as thick as thieves", but my understanding is that it means more like "close" as in "When the two cousins were up to no good they were as thick as thieves". Can anyone help?