Also found in: Dictionary, Thesaurus, Legal.
Related to know from: know from Adam
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.
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?
know from something
to know about something. (Used on the eastern seaboard.) Do you know from thermostats? You don't know from anything!