blind as a bat

Idioms and Phrases

Quite blind; also, unaware. For example, Without my glasses I'm blind as a bat , or I had no idea they wanted me to take over his job; I was blind as a bat . This simile, based on the erroneous idea that the bat's erratic flight means it cannot see properly, has survived even though it is now known that bats have a sophisticated built-in sonar system. [Late 1500s]