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

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?
It's just an idiom.

It's not transitive. I slept in simply means I got up late.
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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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."