means Yellow is the bright color of lemons, sunflowers, and gold — and figuratively, the label slapped on cowardice.

from From Old English 'geolu,' rooted in the Proto-Germanic 'gelwaz' and the Proto-Indo-European base 'ghel-,' which spun off a whole family of words about brightness and the green-to-yellow spectrum — including 'gold,' 'gleam,' and 'glow.' The cowardice sense ('yellow-bellied') is a much later American slang twist, with no clear single source; the popular link to anything biological is just a colorful guess.