the.com/lather
The foam that proves your soap is doing something — or your horse is overworked.
means A frothy mass of bubbles produced by soap, or the sweaty foam on a hard-driven animal.
from From Old English leathor, soda for washing, rooted in a Proto-Indo-European word for washing that also gave us the Greek loutron, a bath.
Horse originSweat foams because horse perspiration contains a natural detergent, latherin.
Worked up'In a lather' means agitated, borrowed from those overheated horses.
Bubbles lieFoam doesn't clean better — manufacturers add foaming agents for vibes.