noun, na-adjective
extremely cautious; taking every precaution
Emphasizes a state of being thoroughly on guard, leaving nothing to chance. Often used in formal or written contexts.
用心堅固な警備体制が敷かれた。
An extremely cautious security system was put in place.
彼は用心堅固に行動し、危険を避けた。
He acted with great caution and avoided danger.
用心深い is a common i-adjective meaning 'cautious' or 'wary', while 用心堅固 is a more formal, emphatic na-adjective/noun meaning 'extremely cautious' or 'taking every possible precaution'.
Compound of 用心 (caution, care) and 堅固 (firm, solid). The exact historical derivation is uncertain, but the combination conveys a sense of solid, unwavering caution.