as blind as a bat
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(as) blind as a bat
1. slang Unable to see at all. After neglecting his vision for years, my grandfather became blind as a bat, unable to even see shadows.
2. slang Having poor vision. I've worn glasses for 20 years, so I'm as blind as a bat now.
3. slang Oblivious to something. Are you blind as a bat? Those two have been flirting all afternoon!
as blind as a bathaving very bad eyesight. informal
This expression probably arose from the bat's nocturnal habits and its disorientated flutterings if disturbed by day. The poor eyesight of bats (and less frequently, moles) has been proverbial since the late 16th century.