(idea) by jliszka Mon Apr 16 2001 at 1:54:14
An abbreviation for "single line only". Used in the subject line of an email to denote that the subject is the entire message, and the recipient need not open the email.

Example: 

To: blau@foobar.com
From: jliszka@foobar.com
Subject: SLO: bring a pen to the meeting tonight
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