What is a rhotic dialect?
A dialect whose speakers pronounce the letter ‘r’ in words.
But don’t we all do that?
Most of us pronounce ‘r’ if it is immediately before a vowel (e.g. in “rat” or “boring”), but speakers of Received Pronunciation (RP) and many other non-rhotic British dialects do not pronounce the ‘r’ if it is at the end of the word (e.g. in “car” or “butter”) or immediately followed by another consonant (e.g. in “party” or “chicken korma”).
Take the word “farmer”. Most of us in England pronounce it “fahmuh”. The only time we pronounce it “farrrrmerrrr” is on Talk Like A Pirate Day. If you pronounce it “farrrmerrrr” in England, people assume you ARE a farmer.
But many international varieties of English are rhotic. In US English, for instance, the ‘r’ in “car” and “party” is usually pronounced.
And some regional British dialects are rhotic. Most Scottish people pronounce every ‘r’ and (although West Country accents are, sadly, slowly dying out) people in the West of England are still more likely to pronounce the ‘r’s in words like “farmer”.
When did English divide into rhotic and non-rhotic varieties?
In the 18th century. There had been a slow tendency towards pronouncing ‘r’ less and less since the 14th century (although not pronouncing your ‘r’s was seen as low prestige), but it suddenly massively accelerated in South-Eastern England in the late 18th century and went from being considered a low prestige feature to being seen as a high prestige one.
Nobody really knows why, but what seems to have happened is this: a small circle of rich aristocrats in London just suddenly decided to stop pronouncing their ‘r’s, on a whim, and it started a fashion. Because the people who had started the trend were of high prestige, everybody wanted to do what they were doing. The more the fashion caught on amongst the upper classes, the more pronouncing your ‘r’s began to seem like a low prestige feature and people who wanted to be thought classy or educated avoided doing it.
But they didn’t have TV or radio or trains or aeroplanes in those days, so the fashion took a long time to travel. The further you were from London and other fashionable big cities, the less likely you were to know about the new trend for non-rhoticity. That is why people in far-flung corners of Britain, like Scotland and Cornwall, didn’t get the memo. (In Scotland, it probably also helped that people wanted to culturally differentiate themselves from the English – they were proud of not talking like the namby-pamby Southerners.)
The fashion also took a long time to get to America. It did catch on a bit there, especially around Boston, but was probably always less widespread than in Britain. From the late 19th century on, rhoticity became considered Standard General American and not pronouncing your ‘r’s is considered low prestige.