Emily Dickinson Lexicon

YEAST, n. [Sax. gist, yeast, a guest, also a storm; yst, a storm; G. gäscht, yeast, and gast, a guest, gäschen, to foam or froth; D. gist, yeast; gisten, to ferment. This coincides with gas and ghost. The primary sense of the noun is wind, spirit, flatulence or froth, from rushing; Ch. גסס, to inflate. Class Gs, No. 18.]

1. Barm; the foam, froth or flower of beer or other liquor in fermentation; used for raising dough for bread or cakes, and making it light and puffy.
2. Spume or foam of water. [Not in use.] – Shak.