A literary, somewhat old-fashioned adverbial phrase meaning to work steadily and diligently without giving up. Often used in formal or written contexts.
He devoted himself assiduously to his research.
His diligent work set an example for those around him.
Variant with repeated kanji; equally rare and literary.
The kanji 孜 means 'industrious' or 'diligent'. The phrase is a classical Chinese-derived expression (孜孜として) used in literary Japanese. The exact historical derivation is uncertain, but it follows the pattern of reduplicated kanji adverbs.