aglow comes from a- ('in a state of') + glow, from Old English glowan ('to glow, shine'), from Proto-Germanic *glo-, from a Proto-Indo-European root *ghel- ('to shine'). Morphologically, the word is transparent as a compound: its parts remain visible in the modern spelling, while the older history of those parts supplies the deeper ancestry. In modern use, the adjective sense is "glowing with warmth, light, or emotion", which keeps the etymology connected to the word's present dictionary value rather than treating it as a merely historical label. The wider word family includes related English forms such as glow, glowing, showing how the same base continues to support neighboring English vocabulary. The principal historical forms in the chain include Old English glowan, Proto-Indo-European *ǵʰel-, which anchor the explanation in named linguistic material.