Womp womp : an expression of sadness or sympathy, spoken in imitation of the noise made by a sad trombone. Often carries a note of exaggerated tragedy.
The Google Ngram database does not include any results for “womp womp,” suggesting that it is little attested in print before 2008. I hear it in my 20–to-40-something social circles in DC.
Thanks to Amanda, who likes to use this utterance.