Firstly, a quick note: I authored an article for Tablet about the first three months of my study of Yiddish. It can be found here.
Now, I wanted to talk a moment to explore the phrase "Dress British Think Yiddish." There is a sort of loose consensus about what it means: Dress mainstream, think outside the mainstream. I think I picked it as the title for my blog because I was born to Irish-American parents and adopted by a Jewish family, and so, even if I didn't already dress in tweeds and flat caps, I still end up looking like someone sculpted a caricature of an Irish guy out of a potato, and so the British/Yiddish thing is especially on the nose in my case.
As far as I can tell, the earliest appearance of the phrase in print was in the Greensboro Daily News on December 24, 1961 from the column "People, Places and Things" by CA Paul. He wrote:
Harry Golden believes it is time for IBM to abandon its motto, "THINK," and suggests a substitute: "Look British, Think Yiddish."
Harry Golden was a newspaper editor and journalist, and years later, in his "The Golden Book of Jewish Humor" he seems to credit himself with the phrase -- I need to track down the book to get the complete story, but it looks like he wrote it in a column and then, literally a few days later, it was quoted back to him by a Hollywood press agent.
A year later, the Jewish Spectator quoted the phrase as the more familiar "Dress British, Think Yiddish," and said it was new. The same year, Commentary offered up the same phrase, and claimed it was dictated by Madison Avenue. In 1964, Show: The Magazine of the Arts credited the phrase to a sign in New York's garment center.
In 1965, the New Jersey State Bar Journal published my favorite version of the phrase, quoting Victor S. Kilkenny: PRESCRIPTION FOR SUCCESS: Look Irish — Dress British — Think Yiddish.
I haven't found the original article, but the Jewish Journal of New Brunswick, New Jersey from Friday, October 22, 1965 ran an excited article summarizing the contents of an Esquire story (A Google search leads me to suspect it's from from Volume 64 - Page 115; maybe I'll try to get to the library soon to look it up). "Esquire Finds Yiddishe Kop on Madison Avenue."
"The article is appropriately titled 'The Yiddishization of American Humor,' written by the humorist, Wallace Markfield," the story tells us, and goes on: "Esquire's author claims (he admits he can't prove it) that the Jewish comic style has invaded Madison Avenue advertising agencies, too. Such slogans as 'Fresher than this there isn't" (for a well-known cream cheese) and "With soap it's loaded" have a definite Yiddish ring to them. And certainly, who could deny that 'Dress British -- Think Yiddish' resembles 'Think British -- Think Byford.'"
Kildare Dobbs, editor of the Canadian general interest magazine Saturday Night, returned the phrase to the garment district in a 1968 story about a schmatte merchant named Hymie. "His creed is simple," Dobbs wrote. "Dress British, Think Yiddish."
By 1969, the phrase was popular enough to become a button, which I own an example of. Venture Magazine mentioned the fact in Volume 6 from that year, mentioning that they are sold alongside buttons reading "Jesus was raised in a kosher home," suggesting that the phrase had been adopted by a inchoate Jewish pride movement.
The Big Apple ran a piece on the phrase back in 2009, and they mention two additional possible sources: First, there was the law firm of Finley Kumble, where Mr. Kumble was famous for advising new layers with the phrase (as well as reportedly banning bagels for being "too ethnic.") But the firm was started in 1968, making it too later to claim ownership. The Big Apple also mentions the financial firm of Salomon Brothers, which supposedly had this as the unofficial policy for their stockbrokers, which included future mayor Michael Bloomberg.
They're a pretty old firm, founded in 1910. But the first mention I find of the phrase being bandied about dates it to 1966, in the book "The Prophet of Love: And Other Tales of Power and Deceit" by Elizabeth Kolbert, and Bloomberg was there the same year, so it is very possible the firm picked up the phrase at the same time as anybody else.
Tentatively, I'm giving credit for it to Golden. I'll see if I can track down his book and will report back. (Note: Tracked down book, and he does indeed take credit for phrase.)