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Don’t know from etymology?

Q: I thought the faux-helpless “don’t know X from Y” phrase was supposed to be a comparison, as in “He don’t know his ass from his elbow.” But I now see it used where only one item is mentioned (“don’t know from X”). The Reader’s Digest website, for example, jokes that its employees were English majors and “don’t know from negatively charged dust particles.” Any comments?

A: This “don’t know X from Y” business is a lot older than you may think.

In fact, the first citation in the Oxford English Dictionary is from Cursor Mundi, a Middle English poem written around 1375: “Mony … knawes nought the gode fra the ille” (or, as we’d put it, “Many don’t know the good from the bad”).

However, most of the OED‘s early citations for this usage are positive, not negative, like this one from Shakespeare’s 1598 play The Merry Wives of Windsor: “We’ll teach him to know Turtles from Jays.”

In the 20th century, the negative construction became more common and often took on a vulgar sense, as in expressions like “He don’t know his arse from his elbow.”

The OED describes this usage as “a coarse expression suggestive of complete ignorance or innocence.”

The first citation for this sense, which uses “ears” instead of “arse,” is from Medal Without Bar, Richard Blaker’s 1930 novel about World War I.

In the book, an enlisted man says none “of us knows ‘is ears from ‘is elbow when it comes to learning like you orficers have got up your sleeves.”

A few years later, two new versions of the expression evolved in the US – this time without the comparison: “know from nothing” and “don’t know from nothing.”

The OED’s first comparison-less citation is from a 1936 issue of Mademoiselle: “I find I belong to the wrong gender to take part in such confabulations, and know from nothing.”

Some language scholars have suggested that these two newer usages were derived from or influenced by similar constructions in Yiddish.

Julius G. Rothenberg cites the double negative version in “Some American Idioms from the Yiddish,” a 1943 article in the journal American speech.

The Jewish Language Research Website, a linguistic site devoted to languages spoken by Jews, says one of the Yiddish idiomatic constructions seen in colloquial English is the “pattern I don’t know from___ (ikh veys nit fun___).”

The Yiddish language maven Leo Rosten, in his book The Joys of Yinglish, says “the ungrammatical substitution of from for about or of” in an English sentence like “What do I know from investments?” is “Bronxian Yinglish” derived from the Yiddish Vos vays ikh fun.

I would describe the usage as idiomatic rather than ungrammatical, but who am I to argue with Rosten about Yinglish?

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