taru-adjective, adverb taking the 'to' particle
Archaic literary word describing a person, attitude, or rule as harshly strict, severe, or unyielding. Used in classical or highly formal contexts; not part of everyday speech.
A passage in a classic text reads 'a very rigorous law'.
His strict and severe manner is said to have made those around him tremble.
The exact derivation is uncertain. The kanji 峭 (steep, severe) and 刻 (carve, strict) combine to suggest a meaning of 'sharply strict' or 'severely rigorous'. The word is archaic and its historical origin is not well documented.