It looks like a parallel to "demnächst". I've never heard the word before. It seems to be a regional word, or maybe some new creation that's slowly making its way into the general vocabulary. http://quake.ingame.de/forum/threads/265465-wer-kennt-das-wort-quot-demletzt-quot Some websites consider it a mistake for "letztens": http://www.korrekturen.de/beliebte_fehler/demletzt.shtml It's not in the Duden.