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During the first week of summer vacation, I slept in every day and watched TV all evening.

Hi,
I presume that "slept in" in the above refer to "got up very late," but why does it mean that? Namely, "slept in what?" Thanks?
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It's just an idiom.

It's not transitive. I slept in simply means I got up late.
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In that expression, I get the sense of "in" as similar to "I stayed indoors" = "I stayed in." = opposite of "I went out." It doesn't exactly refer to "in the house." That is, "I slept in" doesn't necessarily refer to "in the bed."
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Comments  
Yes and no.

It's just an idiom that stands on its own.
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I know you're right, GG. We slept in is not the opposite of We slept out.
And certainly difference from "She slept around."