1. not protected or guarded against attack or harm
2. not supported by argument or justification

Examples

• The castle’s undefended walls made it an easy target for invaders.
• She left her laptop undefended on the park bench.
• The border remained undefended for several miles.
• His undefended thesis was quickly criticized by the committee.
• The small village was completely undefended against the storm.
• The goalkeeper left the net undefended during the corner kick.
• Her undefended argument fell apart under scrutiny.