the word 'yeasty' is a straightforward english formation from the noun 'yeast' and the adjectival suffix '-y'. the noun 'yeast' itself has germanic roots, deriving from old english 'gist' or 'gyst', which referred to the froth or sediment produced during fermentation. this old english term is cognate with similar words in other germanic languages, all pointing back to a proto-germanic source related to fermentation and foam. the modern adjective 'yeasty' thus literally means "full of or resembling yeast," a sense which was metaphorically extended by shakespeare and later writers to describe things characterized by a frothy, insubstantial, or fermentative and energetic quality.

Structure: yeast (substance) + y (adjective suffix) yeast (origin: old english 'gist', 'gyst', meaning ferment, foam) + y (origin: old english '-ig', an adjective-forming suffix meaning "characterized by, full of").