The jūnihitoewas the formal wear of Heian court lady. It translates to “twelve unlined robes”. However the term that was in actual use during the Heian era is karaginumo.
References
The Story of the Kimono by Jill Liddell (1989)
The Pillow Book of Sei Shonagon by Ivan Morris (1991)
The World of the Shining Prince: Court Life in Ancient Japan by Ivan Morris (1964)
Encyclopedia of Hair: A Cultural History edited by Victoria Sherrow