Is the noun liar considered offensive? Would it be offensive to use it with a proven liar?
Calling someone a liar is making a very aggressive statement. If you do so in a bar, don't be surprised if you get a beer bottle upside the head. If you call someone a liar in person or in print, you'd better be able to prove your contention (as @ssaki points out). Either way, don't expect any useful dialogue to ensue after that point.
The noun ‘liar’ is not itself an offensive word in the same way that, say, shit or bitch are — in those cases, the words themselves are (to many people) offensive independently of their meanings. (This is quite silly, as George Carlin probably said best, but it is nonetheless the case.)
With liar, the word is no more offensive or derogatory than its meaning. However, to say that someone is a liar is a real slur on their character: this is the sense in which liar is offensive.
As a native speaker of American English, I would consider it offensive to be called a liar whether it was true or not. You also need to be careful when accusing someone of lying that you don't get into a situation where you are guilty of slander or libel. Even though you consider someone a "proven" liar, the courts may disagree. Laws in your area may differ.
The best way to handling this situation is with diplomacy. For instance, saying,
"I'm not sure that's true."
"What are the facts behind that claim?"
, etc. This way you can put the speaker on the spot to back up a claim without actually calling them a liar. In addition, you may get them to admit what they are saying is not entirely true.
Everyone lies, whether it's a little white lie or a big whopper.
You wouldn't label someone a liar unless they lie habitually.
Calling someone a liar, then, is not making a factual statement about some fib they told last week, but rather is making a judgment about their character, which is why the word carries such a strong connotation and why most native speakers would take offense if you called them a liar.
Liar = One who lies
You said that Santa Claus was real. You told me a lie!
You exaggerated about how much money you make when I asked. You're a liar.
I think English speakers want to be very specific about lies. If called out, I think most would prefer that the particular lie be called out instead of being characterized as a liar. The english language doesn't really specify how many lies a person has to tell in order to become a "liar". Technically, telling one lie makes you a liar, but the word liar has the connotation that you tell lies on a frequent basis, and that is a reflection of your character.
In general, a noun is more offensive than an adjective. (Compare "He is a Jew" to "He is Jewish" or "This house was owned by a couple of gays" to "This house was owned by a gay couple" for example.) I don't know exactly why it is like that, but I guess part of it is that when you stick a noun label on someone you are declaring it the most relevant piece of information, enough to categorize them. The other constructs almost carry a sense of "this is just one piece of information". There is a difference between "you are lying" and "you are a liar" - and almost everyone would be offended by the latter even if they were lying during the conversation in question.
There’s a confusion of cause and effect here. The term “liar” in itself isn’t offensive (unlike, say, “asshole” or other cusswords) – lying is: people have this absurd expectation to be told the truth.
So when you call somebody out as a liar, you lay bare his offense. And since that has such dire consequences, calling somebody “liar” in vain is considered very offensive in itself, because it’s such a serious accusation.
All that just to highlight the difference between words that are meant to be insulting and a word like “liar” which is a statement of facts. It only becomes insulting if it is used to make defamatory false claims.
Define "proven liar."
Without context, "liar" makes me think of someone who lies, usually habitually, for personal gain, often at the expense of others. This might include someone who lies about his experience to get a job for which he's not qualified, or who falsely accuses a coworker of stealing to get that coworker fired.
Calling this kind of person a liar will offend that person, even though the accusation's true. After all, it's hard for him to gain from his lies when people stop believing them.
Will other people be offended? It's hard to say, but it helps a lot if others believe you and feel the accusation's appropriate. Just be aware that "liar" isn't a trivial accusation.
I was called a liar recently and it wounded me. The thing is, the person who called me this does the same behaviors that I am being called a liar about.
If you promise something and do not deliver, I would not call that lying. Some times factors are in play that are out of your control. Like if you promise someone to pay back the $20 you owe them but then you don't get your check right away and have to use the $20 for gas, is that lying? You could in true faith planned to pay the person back when you said. But you just could not at that time.
But taking that large step and calling that person a liar and themselves a fool for believing you makes me feel like it am slandered and he is overdramatizing. Either way it creates a wedge between two otherwise happy people.
So do not call a person a liar unless you are prepared to defend your statement or do not want to have a good relationship with that person anymore. I was called a bitch once in a fight, but it did not hurt as much as being called a liar.
The word liar is not in itself offensive. If someone is on tv and you say "I know that guy, he's a liar" to the mother with 2 young kids, she's not going to ask you to mind your language or drag the kids away from you as she might if you used an offensive word.
Being called a liar is offensive, whether true or not -- possibly more so if it is true than if it isn't, but possibly not.
You call someone a liar when you want to be not just blunt, but rude and offensive.
OTOH, "proven liar" sounds like something you'd say about a politician and would in my view mainly reflect badly upon the speaker. It makes you sound infantile as if the best you can do is dredge up some mistake or misleading statement they made, call it a lie and then try to tar them as a liar.