420

Online jargon, also known as text message shorthand, used primarily in texting, online chat, instant messaging, email, blogs, and newsgroup postings, these types of abbreviations are also referred to as chat acronyms. 

"There are varying theories on the origin of 420: Some say that 420 originated from a police code that announces marijuana use is taking place; another story is that a group of guys (Waldo's) in the 1970's made 4:20 their official meeting time to smoke marijuana after school; and yet another version states that 420 derives from Bob Marley’s birthday, April 20th.

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