To me, having good manners means doing your best to put other people at ease and to be polite in difficult situations.
Apparently, some people think that having good manners is based completely on knowing all the social rules that define a social class - which fork to use, what to talk about/not talk about at dinner, which buttons to do up etc. - and sneer at people who try their best but "aren't quite our sort".
AIBU to think that sneery types are mannerless gits?