know from

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know (someone or something) from (someone or something else)

To be able to distinguish a particular person or thing from someone or something else. Usually used in negative constructions. I've heard his name before, but I wouldn't know him from a stranger on the street. My father is so hopeless when it comes to technology that I'm surprised he knows a mouse from a monitor.
See also: know, something

not know from (something)

To not know anything about something. From Yiddish, heard primarily on the East Coast of the US. Why don't you just stay out of this, OK? You don't know from running your own business. Ah, don't mind her, Phil. She doesn't know from nothing.
See also: know, not
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.

know someone from someone

to tell the difference between one person and another. I don't knowFred from his twin brother. I know Bill from Bob, but I can't tell most identical twins apart.
See also: know

know something from something

to tell the difference between one thing and another. (Often with a negative.) You don't know a smoked herring from a squid! She didn't know a raven from a crow, and who does?
See also: know

know from something

to know about something. (Used on the eastern seaboard.) Do you know from thermostats? You don't know from anything!
See also: know
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.
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