This is a famous sword owned by Uesugi Kenshin and the blade is now Kokuho. It is an Ichimonji sword and has an extremely flamboyant hamon.
The blade is often called Yamatorige or Yamadorige which is one reading of the characters of its name
山鳥毛
These characters can also be read as Sanchōmō though and it’s generally felt that this is the more correct name of the sword.
Does not compute
The weird part though is that the characters in this case spell out Mountain-Bird-Feather and it’s hard to understand exactly why. We encounter this sometimes with named blades, where we have the kanji and have to figure out how it should be read and what the implied meaning is. Given Japanese wordplay it can have multiple poetic implications.
People have thought that maybe the hamon looks like the feathers of a bird…
I spoke with Tanobe sensei about this blade recently and he put forward an interesting theory. He thinks that the name might have been mis-recorded from verbal communication and when it got written down the wrong kanji were used, and we’re maybe then trying to interpret the wrong thing.
All errors on this are mine… I’m going off of memory and I may not have heard it correctly.
Japanese characters can have both the Japanese reading and a reading which is based on their Chinese origin, and this is where we get the reading of Yama and San both for the mountain character 山.
Tanobe sensei said that he felt the blade might have originally been called Sanshōmō which he felt referred to fire on a mountain.
When I got home I spoke with Markus Sesko a bit more about the blade after my conversation with Tanobe sensei and he said:
There exist indeed period transcriptions of that nickname with the kanji 山焼毛 which read Sanshōmō, “mountain — fire — fur/feathers/fuzz”.
I think the oldest entries only have the Yama/San character and the rest in an archaic hiragana transcription of “chōmō”: 山てもう and 山てうまう (てう is archaic hiragana for chō and まう archaic hiragana for mō).The Uesugi records describe the hamon like that, i.e. 山焼毛, Sanshōmō, “resembling a distant mountain range at sunset which looks like being on fire.”
山火毛
Which in Mandarin would be read as Shān huǒ máo which sounds suspiciously close to Sanchōmō.
Given the hamon appearing as if it were flames, I think there is probably some validity in this line of thinking via one approach or another, that the name of the sword refers to “Fire on the Mountain” rather than the plumage of a bird.