"Mi casa es su casa" (formal) is Spanish for "My house is your house" a common saying to invite someone into your house and allow them to make themselves at home. It also carries a implied sexual connotation of granted permission to have intercourse during your stay.
As in Atlas Shrugged(movie):
Ellis Wyatt: There are several guest rooms upstairs help yourself...(evening valedictions)...Mi casa es su casa and so on.
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