means Bread leavened by a live culture of wild yeast and bacteria — a fermented starter kept alive and 'fed' — giving the loaf its characteristic tang.

from A plain compound of 'sour' and 'dough,' from Old English roots — 'sūr' (acid, fermented) and 'dāg' (the kneaded paste). The word names exactly what it is: dough gone tangy. It carried real cultural weight on the North American frontier, where prospectors and Klondike Gold Rush miners guarded their starters so carefully that 'a sourdough' became slang for a seasoned old-timer who'd survived a northern winter — someone, the joke went, who slept with the crock to keep it from freezing.