During a chat with a native English-speaking friend, he used the phrase "get on with it". I asked him about it, and he said it is a slang phrase. I looked it up online, and this phrase shows up on Urban Dictionary, so it seems my friend is correct in saying that it is slang. But who decides whether a word or phrase is proper English or slang?
Everybody, and nobody.
Some countries have formal institutions which lay down rules which state what language is considered proper. English does not. All speakers and writers get a vote, and may exercise that vote as many times as they wish, both by speaking or writing themselves and by hearing and reading and approving what others speak or write. It is the most completely democratic institution in the world.
This is not to say there are no rules. There are, many thousands of them. Language at bottom is nothing but rules.
But the rules govern within larger and smaller communities. The rule which says that the present indicative form of the verb be which is used with the pronoun he shall be is is very widespread, but there are substantial communities where be is employed in some or all circumstances. By contrast, the community which maintains the rule that shall shall be employed in the first person where will is called for in the second and third persons is small and growing continually smaller. And the community which holds that my brother shall be referred to and addressed as Quid is quite small: probably only nine people.
So when you ask this question—which is an important one, and passionately debated—you must also specify ‘correct to whom’? To whom are you speaking or writing?
And when you have specified to whom you will have answered your question. They determine what is correct English.
Slang describes a special kind of vocabulary, so Get on with it isn’t slang. It’s an informal idiom, almost entirely confined to speech. The distinction you have in mind is perhaps not so much between what is slang and what is ‘correct English’ as between Standard English and nonstandard English. In the words of the linguist Richard Hudson,
Roughly speaking, Standard is the kind of English which is:
- written in published work
- spoken in situations where published writing is most influential, especially in education (and especially at University level)
- spoken “natively” (at home) by people who are most influenced by published writing - the “professional class” ’.
Another linguist, Peter Trudgill, has said
Standard English . . . is the variety of English normally used in writing, especially printing; it is the variety associated with the education system in all the English-speaking countries of the world, and is therefore the variety spoken by those who are often referred to as "educated people"; and it is the variety taught to non-native learners. But most native speakers of English in the world are native speakers of some nonstandard variety of the language, and English . . . can be described . . . as consisting of an autonomous standardised variety together with all the nonstandard varieties which are heteronomous with respect to it. Standard English is thus not the English language.
There is no body that says what is Standard English in the way that the French Academy says what is Standard French. If you want to know in what contexts a word or a construction is used a good dictionary will help, or perhaps a corpus-based usage guide such as 'The Cambridge Guide to English Usage'. But there is really no substitute for paying attention to the way in which native English speakers and writers use the language in different situations.
My linguistics prof had a nice buzzword for the standard flavor of a language. He called it the socio-economically preferred dialect.
Who decides what is or isn't in the socio-economically preferred dialect is, why, the socio-economic elite which practices and promotes that dialect, through its media and various institutions.
There isn't a single such a dialect of English because there are numerous English-speaking societies in the world, with their own socio-economic elites with their own dialects.
If we were to identify something as Standard English for the entire world, it would have to be some kind of cmmon subset of all those dialects. Unfortunately, one of the implications would be that some objects would be left without names. For instance petrol appears in the socio-economically preferred dialect used in Britain, but in America's equivalent dialect it is gasoline. So in an international Standard English that is understood by everyone, there can be no word for a highly refined-fuel for spark-ignited internal combustion engines.