not know (someone) from a bar of soap(redirected from didn't know them from a bar of soap)
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not know (someone) from a bar of soap
To be completely unaware of or know nothing about someone; to have never met the person indicated. My girlfriend got really excited when a movie star apparently walked past us, but I wouldn't know him from a bar of soap. Someone I wouldn't know from a bar of soap just contacted me online, claiming to be a distant relative.