also: じんむこのかた
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.
これは神武以来の大改革だ。
This is the greatest reform since the era of Emperor Jimmu.
神武以来の快挙と称えられた。
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.
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'.