Stands for "fiscally conservative socially liberal." Precisely the group left out in the cold in the two-party system of the United States.
Susan: George told me he didn't even vote in 2008! How could he!
Megan: Well, he's a FCSL.
When one writes an email that is intended to have an attachment, but the author forgets to actually attach the document before it is sent.
Joe, here's that excel spreadsheet you requested. See attached.
Fred, you mean "see unattached"? You forgot the excel file.