[bat] 
| 1. | any of numerous flying mammals of the order Chiroptera, of worldwide distribution in tropical and temperate regions, having modified forelimbs that serve as wings and are covered with a membranous skin extending to the hind limbs. |
| 2. | blind as a bat, nearly or completely blind; having very poor vision: Anyone can tell that he's blind as a bat, but he won't wear glasses. |
| 3. | have bats in one's belfry, Informal. to have crazy ideas; be very peculiar, erratic, or foolish: If you think you can row across the ocean in that boat, you have bats in your belfry. |
bat
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blind as a bat
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]