In the "centuries old, but news to me" category, via an economics-oriented listserv I'm on, I today learned the context of Carlyle's famously dubbing economics a "dismal science." See this, from an article in 1849 in Fraser Magazine, called "Occasional Discourse on the Negro Question." Warning: it's ugly.
Truly, my philanthropic friends, Exeter Hall Philanthropy [organized
Evangelicalism against slavery] is wonderful; and the Social Science-not a
"gay science," but a rueful[one]-which finds the secret of this universe in
"supply and demand," and reduces the duty of human governors to that of
letting men alone, is also wonderful. Not a "gay science," I should say,
like some we have heard of; no, a dreary, desolate, and indeed quite abject
and distressing one; what we might call, by way of eminence, the dismal
science. These two, Exeter Hall Philanthropy and the Dismal Science, led by
any sacred cause of Black Emancipation, or the like, to fall in love and
make a wedding of it, -will give birth to progenies and prodigies; dark
extensive moon-calves, unnameable abortions, widecoiled monstrosities, such
as the world has not seen hitherto!'
Economics, then, is dismal because its arguments can be used against slavery. Further historical context on the quote here, from Robert Dixon of the University of Melbourne. Dixon writes: "These then are the true circumstances in which Political Economy (or Economics) was first labelled "The Dismal Science". It is a circumstance we should draw to the attention of our students. They, like us, can be proud to be associated with those economists who were the target of Carlyle's scorn." Commenters, feel free to tell me every educated person in the world already knew all this.
UPDATE: But you know who did know it? Everyone who pays attention to Reason magazine, that's who–see this February 2002 interview by Nick Gillespie with David M. Levy, who wrote a whole book on the context of Carlyle's remarks. So why did I, who actually was a staffer on the magazine when that interview appeared, not know it? To quote Steve Martin, "I forgot."