Back In Five
As in "Back In 5 minutes." You will also see BI10 (Back In 10 minutes), BI15 (Back in 15 minutes), etc., and sometimes this expression is written as BIfive, BIten, etc.
Online jargon, also known as text message shorthand, used primarily in texting, online chat, instant messaging, email, blogs, and newsgroup postings, it is spelled in all lowercase because it's also considered a form of leetspeak.
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NetLingo Classification: Acronyms and Text Message