The sword was named 'Ama no Murakumo no Tsurugi' due to the fact that there were always clouds above the head of Yamata no Orochi (the Eight-Forked Serpent).
ヤマタノオロチ（八俣遠呂智）の頭上にはいつも雲がかかっていたので「天叢雲剣」と名付けられた。