1560s, "not defended, unprotected," from un- (1) "not" + past participle of defend (v.). Attested earlier in a now-obsolete sense of "unforbidden" (late 14c.).
权威例句
1. The coast was undefended and lay open to attack.
该海岸未设防,易受攻击.
2. The fictitious colonel no longer asserted that the island's north shore was undefended.
那位虚构的上校不再申述该岛北岸没有设防了.
3. Bombs and poison gas would rain down on undefended cities.
炸弹毒气弹纷纷投向那些没有防御的城市.
4. Meanwhile, military satellites are completely undefended and vulnerable to attack.
同时, 军用卫星不但易受攻击,而且对攻击毫无抵御能力.
5. Americans have a special relationship with Canadians with whom they share the world's longest undefended border.