also: ふようじん
It's unsafe to walk alone on the street at night.
It's careless not to lock the door, isn't it?
危ない is a general i-adjective for 'dangerous' or 'risky,' while 不用心 emphasizes a lack of caution or security, often due to negligence.
油断 is a noun/suru-verb meaning 'inattention' or 'letting one's guard down,' focusing on the mental lapse; 不用心 describes the resulting unsafe state or careless action.
不用心 describes being careless or leaving oneself unguarded against danger, the direct opposite of being 用心深い.
Standard kanji spelling; also read ふようじん by some speakers.
Alternate reading restricted to the 不用心 spelling; ぶようじん is far more common.
Variant kanji spelling with 無; less common but seen in older texts.
Compound of 不 (negation) + 用心 (care, caution). The spelling 無用心 uses 無 (without) for a similar meaning. The reading ぶようじん is a rendaku form of ふようじん.