it means murder/ homicide

187 is the the penal code for murder/ homicide. Online jargon, also known as text message shorthand, used in texting, online chat, instant messaging, email, blogs, and newsgroup postings, it is also considered a form of leetspeak because it is code using numbers.

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See also : 143  182  404  
NetLingo Classification: Acronyms and Text Message

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