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1.
a dead letter is a letter that has never been delivered because the person to whom it was written cannot be found, and it also cannot be returned to the person who wrote it
if u send a letter and the person dont get it and u dont get it back
by gashford April 09, 2004
 
2.
In a blog or other community forum, the action of writing or posting comments which serve to create a fuss or controversy, but do not help resolve the issue being discussed. When you "deadletter" a public discussion space, you kill the opportunity for effective discourse on relevant subjects and kill the possibility of progress on those issues by diverting attention and energy to tangential or non-sequiteur issues, esp. using incendiary, passive-aggressive, or thinly-veiled remarks aimed at personal or public/political targets.
"That guy totatally deadlettered the issue at hand." (or) "Please ignore that deadletter post about the upcoming event; the important issues we need to cover before the event are really these..."
by Gratidude September 25, 2007
 
3.
A letter that has died.
E - "Did you hear that R has passed away?"

Jonny Woss - "Weawy? I didn't like him much anyway."

Hence the dead letter.
by leeful November 16, 2006