Learn English – Origin of the term “fat chance”

How about "thousands" of instances of its usage? I can't resist the irony! (Add your contribution!)

• If I've told you once, [please consider] I've told you a thousand times. - just now
• If I've Told You Once, I've Told You 1,000,000,000 Times - 2010
• If I've told you once, I've told you countless times - 2002
• Dear Rocker in Constance: If I've told you pinheads once, I've told you a thousand times — anybody who listens to the Beatles in 1995 is not only living in the past, he's brain dead! - 1995
• I told you once, Hopkins, I told you million times - 194?
• If I've told you once I've told you five thousand times - 1941
• If I have told you once I have told you ten dozen times - 1930
• If I've told you that once, I've told you thrice — I've told you a thousand times - ?1927?
• don't you know that I not only told you once but, in response to repeated importunities from you, that I told you several times - 1919
• if I said to you once, don't touch confiscated property, I told you so ten thousand times - 1866
• once is as good as if I had told you a thousand Times over - 1735

As a @FumbleFinger's once said:

It's just a standard speech device that people reinvent repeatedly

In "Glossary of supposed Americanisms" by Alfred Langdon Elwyn (1859), I find this:

Let on, to mention. "He never let on," he never told me. An Icelandic word, laeta. (Brockett.) "He never let on to me about it," may be heard not unfrequently in the country.

The "Brockett" referred to appears to be "A Glossary of North Country Words", by John Trotter Brockett, FSA (1846) but I can't find the reference to "let on" or "laeta" in the samples that Google provides.

The first place I've found it used with the implication of "lying by omission", as in your example, is in a story called "Little Fairly" by Samuel Lover, Esq. R.H.A. in "The Dublin University magazine", Volume 1 (1833)

"Well there's the scales for you," says big Fairly, and away the little chap wint to weigh his goold (as he let on) as he did before.