盗聴器 [とうちょうき] – bug; wiretap; a listening device

• 盗 [トウ・ぬすむ] – to steal; to rob
• 聴 [チョウ・きく] – to listen; to inquire
• 器 [キ・うつわ] – device; instrument

I love how (some) Japanese compound words work.  A bug is a device that allows you to steal listening.