also: じんむいらい
noun which may take the genitive case particle 'no', adverb
since the era of Emperor Jimmu; unprecedented in Japanese history
Used to emphasize that something is the first of its kind in Japan's recorded history, often in formal or journalistic contexts. The phrase is fixed and not used as a general adverb.
This is the greatest reform since the era of Emperor Jimmu.
It was praised as an unprecedented achievement in Japanese history.
未曾有 means 'unprecedented' in a general sense, while 神武以来 specifically ties the unprecedented nature to the entire span of Japanese history.
Standard kanji spelling; the reading じんむいらい is the most common.
Alternate reading using このかた; less common but still encountered.
Literally 'since (the time of) Emperor Jimmu', referring to the legendary first emperor of Japan. The phrase is used idiomatically to mean 'unprecedented in Japanese history'.