under the counter
under the counter {adv. phr.}, {informal}
Secretly (bought or sold).
That book has been banned, but there is one place you can get it under the counter.
The liquor dealer was arrested for selling beer under the counter to teenagers.
- Also used like an adjective, with hyphens.
During World War II, some stores kept scarce things hidden for under-the-counter-sales to good customers.
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Source: A Dictionary of American Idioms