An expression should be considered "true slang" if it meets at least two of the following criteria:
- It lowers, if temporarily, "the dignity of formal or serious speech or writing"; in other words, it's likely to be considered in those contexts a "glaring misuse of register."
- Its use implies that the user is familiar with whatever is referred to, or with a group of people who are familiar with it and use the term.
- "It's a taboo term in ordinary discourse with people of a higher social status or greater responsibility."
- It replaces "a well-known conventional synonym". This is done primarily to avoid discomfort caused by conventional phrases or by further elaboration.