Circa 1999, Brenda designed a tshirt for the PMAMC&OC at UW which read:
Women shouldn’t be worrying their pretty heads about geometry.
— Immanuel Kant
Underneath was a disclaimer:
NOTE: The Pure Math, Applied Math, and Combinatorics and Optimization Club of the University of Waterloo does not necessarily endorse any baseless, asinine claims made by dead German philosophers.
I wore this shirt recently to Nikita and Lenore’s wedding in Toronto, a wonderful and very geeky affair, where it was big hit with the crowd. One woman asked whether the quotation was apocryphal, however, which made me realize I didn’t know its source.
Wanting to be able to fully stand behind my tshirts, I googled it, then wrote Brenda, who told me:
I got the quote, secondhand, from William Dunham’s book The Mathematical Universe - it may be a paraphrase of a translation, which is why Google didn’t produce anything.
I dug around using the full text search on amazon.com, and found the relevant quote from Dunham’s book (first published in 1994):
Immanuel Kant is said to have remarked that women might as well have beards as “worry their pretty heads about geometry”. [p. 294]
I’m guessing Dunham heard this story at a cocktail party or something, as he threw in the caveat ‘is said to’. But the bit about beards eventually led me to “Merging and Emerging Lives: Women in Mathematics” by Claudia Henrion, published in Notices of the AMS in 1991. She wrote:
Kant continued in this tradition, defining math as the realm of men, saying that “women should not worry their pretty heads about geometry—that they might as well have beards”.
Her citation noted that this was not a direct quotation, but rather a paraphrase from Immanuel Kant’s Observations on the Feeling of the Beautiful and the Sublime (1764), a work on classical aesthetics. Aesthetics is the theory or philosophy of taste, an attempt to quantify the beautiful in nature and art, and being a set of ‘scientific’ rules for why people like or dislike art is more clearly utter claptrap than most classical philosophy. Thanks to books.google.com I found the relevant passage in the third section, “Of the Distinction of the Beautiful and Sublime in the Interrelations of the Two Sexes”, where Kant writes:
The fair sex has just as much understanding as the male, but it is a beautiful understanding, whereas ours should be a deep understanding, an expression that signifies identity with the sublime. To the beauty of all actions belongs above all the mark that they display facility, and appear to be accomplished without painful toil. On the other hand, strivings and surmounted difficulties arouse admiration and belong to the sublime. Deep meditation and a long-sustained reflection are noble but difficult, and do not well befit a person in whom unconstrained charms should show nothing else than a beautiful nature. Laborious learning or painful pondering, even if a woman should greatly succeed in it, destroy the merits that are proper to her sex, and because of their rarity they can make of her an object of cold admiration; but at the same time they will weaken the charms with which she exercises her great power over the other sex. A woman who has a head full of Greek, like Mme Dacier, or carries on fundamental controversies about mechanics, like the Marquise du Châtelet, might as well even have a beard; for perhaps that would express more obviously the mien of profundity for which she strives. [p. 78; emphasis in original]
So, the quote on my tshirt is a rather loose paraphrase, but by no means a misrepresentation. (I had to look up ‘mien’. It means ‘dignified manner or conduct’.) But who was the Marquise du Châtelet? It seems Émilie du Châtelet (1706 - 1749) was a noted French mathematician and physicist, who among other feats learned English to write the first French translation of Newton’s Principia Mathematica (hers is still the standard French edition), demonstrated that the kinetic energy of an object is a function of the square of its velocity, and correctly predicted infrared radiation. I must confess I had never heard of her, perhaps because of the tendency for women’s contribution to science to not so much be struck from history as never acknowledged in the first place. Voltaire, one of her many lovers, thought her brilliant and wrote that she was
a great person whose only fault was being a woman. I’ll leave the last word to her:
Judge me for my own merits, or lack of them, but do not look upon me as a mere appendage to this great general or that great scholar, this star that shines at the court of France or that famed author. I am in my own right a whole person, responsible to myself alone for all that I am, all that I say, all that I do. It may be that there are metaphysicians and philosophers whose learning is greater than mine, although I have not met them. Yet, they are but frail humans, too, and have their faults; so, when I add the sum total of my graces, I confess I am inferior to no one. 
All I can add is that it’s lucky for Kant she died young before he got around to ridiculing her, or he and his baseless, asinine claims would have had it coming.