What kind of phrase is to spend the night?

Answer 1

This is called a "euphemism." (When you see the prefix "eu", it means good or pleasant. A euphemism is a good or pleasant way of saying something the society might regard as rude or vulgar.) In older literature, referring to "spend the night" often implied that the person was going to engage in sexual relations, but this could not be said, as it was considered vulgar, so a euphemism was used. To this day, there are many euphemisms for sexual relations, parts of the body, and bodily functions, as well as for concepts that the culture would rather not come right out and say -- for example, to "pass away" rather than to die.