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1. To eat at one's home, rather than at a restaurant. In this usage, the phrase is always "eat in." Neither of us felt like going out, so we ate in tonight.
2. To erode or wear something away. Don't use that harsh cleaner—it will eat into a soft metal like this.
eat in (to something)
to erode into something; to etch something. The acidic water ate into the rocks on the shore. The acid ate in and weakened the structure.
to eat a meal at home. I really don't want to eat in tonight. Let's eat in. I'm tired.
Have a meal at home, as in Are we eating in tonight? [Colloquial; second half of 1900s] Also see eat out, def. 1.
To eat at home, rather than in a restaurant: It's raining outside, so let's eat in tonight.