"Slang" can be defined as 'language peculiar to a particular group", so therefore, it can be very difficult to understand and even more difficult to learn for yourself. So how does one decipher all this slang, when you are not native to the area and you are unfamiliar with this jargon? It's probably not as hard as one may think.
People once new to the U.S. will tell you that they learned slang by reading newspaper columns that had been in existence for many years: 'Dear Abby" or "Ann Landers", where readers would write to these erudite twins for advice on everything from marriage to grief. With their quips, barbs, and fast-talking comments, readers would be able to make sense of the meaning of these words, based on the context of the question and the following reply from the advice columnist. There are many other ways to pick up on American slang.
The internet is probably our most valuable tool in today's society to inform oneself on any matter. Websites such as YouTube, with its phenomenal popularity, can have you hearing the tone in the speaker's voice and understanding the meaning while seeing the reference that is being made. Hear a term you are unfamiliar with? You can Google it - and there's a free slang term for you, as 'google' has now become a verb in its own right!
Spending time with teens will certainly have you learning slang in no time! Slang is the language of American teens. Even American parents are often unsure of what their own teens are talking about when they are in a group of their friends or addressing the parent. A translation is often needed. Slang is an ever-changing language that can be very in vogue one week and be obsolete the next--or very 'uncool'.
Television is always an excellent resource if you're trying to get a grasp on slang. From the highly-viewed reality shows to talk shows, slang is ever-present. Movies seen at the theater are also an excellent source of slang, as it is so pervasive in the American lexicon. The less cerebral movies will likely have more slang as slang is not always perceived as favorably among the intellectuals nor in the workplace. It is casual in style and portrays a sort of laziness in speech that is not always warmly received. Slang is mostly used in circles of friends and at school, and certainly not while visiting Grandma or on a job interview.

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