A person (e.g. your brother-in-law) who enters your house without being invited, opens your fridge without asking, etc. Not just "impolite" of course, something more specific and informal
overfamiliar [Farlex] - taking undue liberties
b. Unduly forward or brash; offensively presumptuous: She displayed an overfamiliar attitude toward her superiors. [AHDEL]
In the given example, I would find both the behaviour, and the person committing it, to be presumptuous:
(Of a person or their behaviour) failing to observe the limits of what is permitted or appropriate
However, this could sound somewhat reserved, and definitely doesn't have any bite. As the question is a request for something specific and informal, my reflexive response to somebody taking it upon themselves to start rifling through my fridge would be that they're taking the piss:
to take liberties at the expense of others, or to be unreasonable.
This is a British (and I think Australian) idiom, so I don't know if Americans and other English speakers would understand the meaning. It is vulgar, so if it's your priest or solicitor who is scoffing your Dairylea Triangles, you might not want to resort to this; however it's not overly rude, and could certainly be used amongst friends, family and even colleagues with whom you have a good relationship.
Having said that, somebody who marches into your house and begins helping themselves to your food really is taking the piss, so I personally wouldn't be too worried about offending such a misanthropic slob.
In British English I'd say:
- He's got a (lot of) nerve, helping himself like that.
- Fig. great rudeness; a lot of audacity or brashness. (*Typically: have ~; take ~.)
He walked out on her, and that took a lot of nerve!
You have a lot of nerve! You took my parking place!
What (a) nerve! and Of all the nerve!
Inf. How rude!
- Bob: Lady, get the devil out of my way! Mary: What a nerve!
- Jane: You can't have that one! I saw it first! Sue: Of all the nerve! I can too have it!
Impertinent boldness: had the cheek to insult his hosts
(U.S. informal) I'm amazed they have the cheek to ask in the first place
- He's got a cheek coming to my home without being invited.
Other informal synonyms are:
Discourteous seems appropriate.
rude or impolite : not showing good manners
showing rudeness and a lack of consideration for other people.
You could also consider insolent, though that may be harsher than necessary in my opinion.
boldly rude or disrespectful; contemptuously impertinent; insulting
Rude, ill-mannered, or just plain impolite should also suffice.
In Chinese culture, we normally describe people exhibiting this behavior as 没家教, which roughly translates to poor family upbringing, and reflects badly on the individual's parents.