gʰeys-
“to be in fear, to be struck with terror”ghost, ghastly, aghast
Root of Old English gāst "spirit", yielding English ghost, ghastly, aghast.
Discussion
The Proto-Indo-European root *gʰeys- meant "to be in fear, to be terrified, to be struck with amazement" and produced a small but evocative family of descendants concentrated primarily in the Germanic languages. The root captures the primal experience of dread and supernatural terror — emotions that were closely linked in Indo-European thought.
In Germanic, the initial aspirated voiced stop *gʰ became *g, and the root produced Old English gāst "spirit, soul, breath" — the ancestor of modern English ghost. The original meaning of ghost was not primarily a revenant or phantom but rather a spirit or animating force, as preserved in the phrase "Holy Ghost" and in the archaic sense of "giving up the ghost" (dying, releasing one's spirit). The modern sense of "a dead person's apparition" developed during the Middle English period, gradually displacing the older, broader meaning.
The word aghast is a related derivative, from a Middle English form meaning "terrified, struck with horror" — literally "ghosted" or "spirit-struck". The word ghastly (originally "frightening, causing terror") also descends from this root, as does the Scottish gast "to frighten".
German Geist "spirit, mind" is the most significant cognate, carrying a broader philosophical meaning than English ghost — it encompasses intellect, spirit, and zeitgeist (the "spirit of the age"). The difference between English ghost (narrowed to spectral apparitions) and German Geist (broadened to encompass mind and culture) illustrates how cognate words can drift in opposite semantic directions.
Outside Germanic, the root is poorly attested, leading some scholars to question its Proto-Indo-European status and suggest it may be a Germanic innovation. However, possible connections to Sanskrit hīḍ- "to be angry" and other forms have been proposed.
The root *gʰeys- offers a glimpse into Proto-Indo-European psychology — the intimate connection between terror, spiritual experience, and the concept of the animating breath that distinguishes the living from the dead.