Borrowed from English cakes, 1900s. Popularized by German food company Bahlsen which changed the brand name of their biscuits from Leibniz-Cakes to Leibniz-Keks in 1911; at the time, the plural was also Keks. Compare Danish kiks, Estonian keeks, Faroese keks, Finnish keksi, Icelandic kex, Norwegian kjeks, and Swedish kex, all ‘cookie, biscuit, cracker’.