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The act of sending a tweet in which the user reflects on a sentimental moment of their life, i.e. a graduation, the end of a life period, or the passing of a loved one.
"OMG No longer in high school!!! Gonna miss it but moving on to bigger and better things!"

"RIP Grampa Joe. You lived a great long life but I'm glad you're in a better place now."

"All these bittertweets are bumming me out, man."
by SamSJS June 20, 2012
To post a negative tweet from the web site Twitter.
After John's movie was given a poor review he quickly posted a bittertweet against the author from his Twitter account.
by Zebjr September 30, 2009