Like the F-, the F-- is the worst grade possible. However, an F- can be given for an assignment that wasn't turned in or had little or no effort put into it. An F-- not only denotes an awful grade, but signals that the content, no matter how much work was put into it, was so awful that a mere F- wouldn't be painful enough for its recipient.
The boy was stunned when he received an F-- on his paper; he had stayed up all night working on it.
by Diggity Monkeez February 24, 2005