The risk with slang is not that nobody will follow you. It is that everybody will, and that the word will place you: in a decade, in a country, and in an age group, none of which you picked.
So the useful thing to know about a slang word is never what it means. The meaning takes four seconds to find. What matters is where it sits on the range between something you could say to anybody and something that identifies you the moment it leaves your mouth, and no dictionary records that.
Eight to recognise, and one rule about using them
Slang is regional, generational and dates quickly. Understanding it matters; producing it as a learner rarely lands.
- take slang in, do not put it out. Slang used slightly wrong or slightly out of date is more conspicuous than formal English, and the risk is asymmetric.
- knackered, exhausted, British and very common.
- gutted, bitterly disappointed, and it is stronger than it sounds.
- chuffed, pleased, and it is the opposite of the previous one.
- dodgy, unreliable or slightly suspect, applied to food, people and wiring alike.
- sorted, dealt with, and it works as a whole reply on its own.
- mate, friend, and also a way of addressing a stranger, which can be warm or cold depending entirely on tone.
- a quid, one pound sterling, and it has no plural: five quid rather than five quids.
Every word is somewhere on the range, and slang is only the far end
Register is not a switch between formal and informal. It is a continuum, and the practical skill is not avoiding one end of it but knowing roughly where a given word sits, which is a thing you can be told about a word you already use. Children, kids and half a dozen slangier alternatives all mean the same and put the speaker in different rooms.
Once you think of it that way, a slang word stops being a special category with its own rules and becomes an ordinary vocabulary item that happens to sit at one end. It gets stored with a note about where it sits, exactly as a technical term gets stored with a note about what field it comes from.
Where it stops: everything learned from something already old
Almost every learner acquires slang from series and films, and this is the one part of the vocabulary where that source is actively misleading. A series was written, filmed and released over a couple of years, and it is usually watched some years after that, so a word arrives already several years old. Slang turns over fast enough that several years is the difference between current and dated.
The stall is that this feels like the safest possible source. It came from a native speaker, in a real sentence, said naturally. The learner produces it with confidence, gets understood, and is placed in a decade they have never lived through, and because the sentence worked nobody says anything. Every check the learner has available reports success.
The same trap has a country in it. A great deal of what reaches a learner comes from American production and a great deal from British, and mixing them inside one paragraph is more noticeable than either one on its own.
The reference, and how far to trust it
Urban Dictionary is where these words are actually recorded, and it is free with no account. For a phrase you have just heard and cannot place, it usually works, which is more than any published dictionary manages, because it is written by the people using the words rather than by lexicographers waiting for a term to settle. It replaced the printed dictionary of slang, which was out of date before it reached a shelf.
Now the part that has to be said plainly. Anyone can submit an entry, many entries are jokes, and definitions are voted on rather than checked. There is no date you can rely on, no note on region, and no indication of how strong a word is. It is a reference for reading and for working out what somebody meant. Treating a hit there as permission to say something is the single most reliable way to produce a sentence you will regret.
What it looks like when it goes wrong
The recognisable failures are all social rather than linguistic. A dated word, which reads as a costume. A word belonging to a group the speaker is visibly not part of, which reads as borrowing. A word from one country dropped into a paragraph of another, which reads as a patchwork.
And one that is worth naming on its own, because learners are consistently wrong about it in the same direction: English swearing is stronger than its translation. The word your bilingual dictionary offers as an equivalent is almost always milder in your own language than the English one is, so a word you have used lightly since childhood arrives in English carrying weight you did not intend. Nobody will explain this to you. They will simply revise their view of you.
The slower-moving version of the same placement problem, where a phrase can be twenty years out of date rather than three, is phrases that do not mean what their words say, and both sit under building the word count you need to say what you mean. Because the whole recommendation here is comprehension rather than production, the skill this actually depends on is real speed, real accents, no subtitles.
Which leaves the question with the highest stakes attached to it. Everything above is about being placed socially, and there is one room where being placed wrongly costs something specific and immediate rather than something vague: the English that decides whether you get the role.