thick as thieves

phrase

: very close and secretive

They were (as) thick as thieves for weeks, which made us wonder what they were doing.

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“Thick as thieves.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/thick%20as%20thieves. Accessed 14 Mar. 2026.

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thick as thieves

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