That is one of my pet peeves too. It's rude, it's dismissive, and basically says that the person wasn't really listening in the first place and thinks that whatever they have to say is more important than anything YOU could POSSIBLY ever say.
I know what you mean, but some people will be rude. Interruptions are generally accepted in non-formal settings though. While I get annoyed too, people aren't going to change and one needs to learn to live with rudeness.
They are rude. You should continue speaking but first say" As I was saying" then continue. If it reoccurs you could take it up a notch by saying "I was raised to not interrupt a lady or gentleman when they were speaking You my friend sadly were not as well raised and I understand. "
I rarely interrupt anyone believing that courtesy and good manners should prevail. However, depending on circumstances, I may cut someone off when: 1. They are rambling, and interrupting them is the only way to get back on track. 2. They are missing the point, and I interrupt them to clarify or correct their misperception. 3. The point they are making is so obvious that everyone else in the room got there without the need to make it explicit. Allowing the person to finish will just waste time.
It's obviously not true in all instances, but sometimes while one is speaking another person may actually be daydreaming then suddenly butt in without thinking. This most often is the case when the subject matter of the butt-in is not relevant to what you were saying. Not everyone is quite as rude as they seem. But, yes, I know what you mean.
put your fingers to your ears and when the idiot ask what you are doing..tell them you aren't interested in what they have to say until AFTER you are through yapping!! and furthermore, you may not be interested THEN!...