na-adjective, noun which may take the genitive case particle 'no'
defenseless; unprotected; vulnerable
Describes a state of being without defense, protection, or guard, often implying a lack of preparedness or caution. Can be used for physical, emotional, or metaphorical vulnerability.
無防備な状態で敵に攻め込まれた。
We were attacked by the enemy in a defenseless state.
彼女は無防備に秘密を話してしまった。
She carelessly revealed the secret, leaving herself vulnerable.
脆弱 is more formal and often used for systemic or structural weakness, whereas 無防備 is broader and can describe temporary or personal states.
Compound of 無 (mu, 'without') and 防備 (bōbi, 'defense, protection'). The word 防備 itself is composed of 防 (bō, 'defend') and 備 (bi, 'prepare').