know from a bar of soap

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know from a bar of soap (third-person singular simple present knows from a bar of soap, present participle knowing from a bar of soap, simple past knew from a bar of soap, past participle known from a bar of soap)

1. (UK, Australia, New Zealand, South Africa, idiomatic, informal) To know; to be acquainted with (a person).

   After she won the lottery, Marge had long-lost relatives she didn't know from a bar of soap come up to her to ask for money.

This expression is more commonly used in its negative form.