hole-in-the-wall
hole-in-the-wall {n. phr.}
A small place to live, stay in, or work in; a small, hidden, or inferior place.
The jewelry store occupied a tiny hole-in-the-wall.
When Mr. and Mrs. Green were first married, they lived in a little hole-in-fhe-wall in a cheap apartment building.
2. {slang}, {citizen's band radio jargon}. A tunnel.
Let's get through this hole in the wall, then we'll change seats.
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Source: A Dictionary of American Idioms