What is yungdrung?

"One of the most powerful and resonant words in pre-Buddhist Tibet was yungdrung (g.yung drung). It was a the key terms for the old royal religion, the mythological backdrop to the kingly lineage of the Tibetan Empire. For example, the inscription of the tomb of Trisong Detsen has the line: “In accord with the eternal (yungdrung) customs (tsuglag), the Emperor and Divine Son Trisong Detsen was made the ruler of men.” I discussed how to translate that term tsuglag in an earlier post. Here, as you no doubt noticed, I have translated yungdrung here as “eternal”. Eternity seems to be the general meaning of yungdrung in the early religion. In addition, the word was associated with the ancient Indo-European swastika design, which in Tibet was the graphic symbol of the eternal."

http://earlytibet.com/2008/04/30/buddhism-and-bon-iii-what-is-yungdrung/