clabber; sour milk that has thickened and curdled.
Etymology: from Irish Gaelic clabar, mud (similar to Irish claba, thick). Short for bonnyclabber (Irish bainne, milk).
In the late 1700's (or possibly earlier), inventive cooks experimented with methods of leavening (adding air to) dough without using yeast. An acid-alkali chemical reaction could quickly add bubbles - useful for making "quick breads". Clabber could provide the acid part of the reaction; potash might be used as the base.
Modern baking powder was invented in 1843 by Alfred Bird; it uses the same sort of acid-base chemical reaction but avoids the strong flavor of potash and the time required to curdle milk. Baking powder is composed of phosphate and baking soda (bicarbonate). Hulman & Company changed its baking powder product's name to Clabber Girl in 1923.
Traditional clabber is made using raw milk, a week or two old, that is left out in a warm area for a few days until solids appear. Pasteurized milk doesn't contain the needed bacteria for the process, but raw milk can potentially contain some very dangerous bacteria. Clabber can also be made by combining milk with vinegar or rennet. Another option is to use pasteurized milk and buttermilk.