noun
great depth; great height; 10000 fathoms
Literary or classical term for an immeasurably deep or high place. Often used figuratively to emphasize extreme depth or height.
彼の学識は万尋の深さがある。
His scholarship has the depth of 10,000 fathoms.
万尋の谷底から見上げる空は小さかった。
The sky looked small when looking up from the bottom of the unfathomably deep valley.
千尋 means '1000 fathoms' and is used similarly for great depth, but 万尋 implies an even greater, almost infinite scale.
Compound of 万 (ten thousand) and 尋 (fathom, a traditional unit of depth). The exact historical derivation is uncertain, but the combination conveys an immeasurably large depth or height.