In the distant past, I always figured slang was nothing more nor less than peculiarities of generations--  kids using words in different ways to set themselves apart from their elders.  Later, I also found slang words, terms, and various colloquialisms are also common to various regions.  In both cases, the general idea is for people to be understood by other people who are like themselves.  

Upon arriving in this city, I immediately encountered a language barrier.  The first example was when a specific item did not arrive in the newspaper, and I went to a local store to ask if they had any.  Sales papers, circulars, flyers, ads--  the clerk could not grasp what I meant.  A second example came after I had attended a barbecue, and offered a bag to a neighbor.  Without asking what the food was, he said he never eats barbecue.  It was not til I found a website that listed the different ways words and terms are used in different parts of the United States that I could figure out both the neighbor's confusion and mine--  in this area, "barbecue" is a type of food, whereas where I am from it's a method of cooking.  

The language barrier goes way beyond regional expressions, and is not limited to any one age group or generation.  I have found even individuals who are considerably older than myself use the vocabulary of the "Urban Dictionary."  I cannot believe this is what they were taught in their school days.  Curiosity getting the better of me, I took some of the terminology to "Google."  One thing I have learned about Google is there is no topic or question that is so unusual or so dopey that someone else hasn't asked it before;  consequently, I found plenty of people asking the same questions.  Even more important--  in my opinion, anyway--  I found plenty of people disagreeing and arguing about it.  There was not one definitive answer on what any of the words and terms meant.  

Expressions can be different between generations, and in different regional areas.  However, people who claim words mean what they want them to mean clearly miss the entire point of language.