yeast (n.) Look up yeast at Dictionary.com
Old English gist "yeast, froth," from Proto-Germanic *jest- (source also of Old Norse jastr, Swedish jäst, Middle High German gest, German Gischt "foam, froth," Old High German jesan, German gären "to ferment"), from PIE root *yes- "to boil, foam, froth" (source also of Sanskrit yasyati "boils, seethes," Greek zein "to boil," Welsh ias "seething, foaming").