Is it ensure, insure, or assure?
early 14c., "leavened bread," also "leaven" (late 14c.), from sour (adj.) + dough. Meaning "fermented dough" is from 1868. The meaning "Arctic prospector or pioneer" is from 1898 Yukon gold rush, from the practice of saving a lump of fermented dough as leaven for raising bread baked during the winter.