1 : of, relating to, or resembling yeast
2 a : immature, unsettled b : marked by change c : full of vitality d : frivolous
I was feeling a little disconnectedness between these two definitions; luckily the etymology elaborates on this a bit:
"...a number of extended, figurative senses of "yeasty" have surfaced, all of which play in some way or another on the excitable, chemical nature of fermentation, such as by connoting unsettled activity or significant change."
The root of the word comes from the Old English gist, gyst, and the Indo-European yes, meaning boil, foam, or bubble—hence the second definition.
I suppose I never thought of yeast as inciting so much change or movement or action, but this makes sense. That said, I admittedly feel a little uncomfortable referring to something as YEASTY, considering what the term means to most women. When I say or hear the word YEASTY, I immediately associate it with having or empathizing with one inflicted by the ever-pleasant vaginal candida (aka thrush).
I have endured three yeast infections in my life, all unpleasant, during none of which at any point would I have referred to myself, or my vagina, as "full of vitality." But this just adds to the diversity of the word YEASTY!
The definition could (should) expand even further to encompass contemporary reference to the yeast-inflicted ailment:
3 : of or relating to the physical symptoms of candidiasis: itching, burning, soreness, irritation, and (what many refer to as) curd-like discharge
M-W, take a hint and nod to the ladies, albeit the grossness.