1
general::
Have a meal at home, as in Are we eating in tonight? [Colloquial; second half of 1900s] Also see EAT OUT, def. 1.
American Heritage Idioms
2
general::
Phrase(s): 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., Phrase(s): eat in
to eat a meal at home. • I really don’t want to eat in tonight. • Let’s eat in. I’m tired.
McGrawhill's American Idioms And Phrasal Verbs