also: ふようじん
na-adjective, noun
unsafe; insecure; careless; dangerous
Describes a situation, place, or behavior that is not safe or secure, often implying a lack of caution or vigilance. Used for physical security as well as personal carelessness.
夜道を一人で歩くのは不用心だ。
It's unsafe to walk alone on the street at night.
鍵をかけない
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.
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.