yeast: [OE] Yeast is etymologically a substance that causes ‘fermentation’. For its ultimate source is the Indo-European base *jes- ‘boil, foam, froth’, which also produced Greek zeín ‘boil’ (source of English eczema) and Welsh iās ‘seething’. Its Germanic descendant produced German gischt ‘yeast, froth’, Dutch gist, gest ‘yeast’, and English yeast. => eczema[yeast etymology, yeast origin, 英语词源]