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| Subject | English | Russian |
| gen. | a deep dark secret | за семью замками |
| gen. | a deep secret | глубокий секрет |
| gen. | a deep secret | глубокая тайна |
| idiom. | deep, dark secret | скелет в шкафу (A piece of information that is extremely private and confidential, usually implied to be embarrassing, incriminating, or shameful: I've carried my fear of clowns as a deep, dark secret for many years now. Many saw her as a perfect candidate, but no one could have known she had a deep, dark secret from her past. Bullfinch) |
| gen. | deep secret | строгая тайна |
| gen. | deep secret | большой секрет |
| gen. | deep secret | непостижимая тайна |
| Makarov. | keep something a deep secret | хранить что-либо в строгой тайне |