What is the word for using someone's hospitality unashamedly? [duplicate]

This so-called "guest" is presuming on his hapless victim host. In other forms, this person is said to be, presuming on someone's generosity, or just presumptuous.

Among the definitions of presume at dictonary.com you will find

to act or proceed with unwarrantable or impertinent boldness

Also, among the Yiddish words we presume to use in English is Schnorrer, which I once heard described as "someone who barges into your house and throws open the refrigerator".

A common term is freeloader

A person who takes advantage of others' generosity without giving anything in return.

Oxford Dictionaries Online

Freeload is similarly defined in Merriam-Webster

to impose upon another's generosity or hospitality without sharing in the cost or responsibility involved

Another informal term is sponge

informal [no object] Obtain or accept money or food from other people without doing or intending to do anything in return.

they found they could earn a perfectly good living by sponging off others

Oxford Dictionaries Online