What semantic notions underlie 'anger, agitation' (PIE *ǵʰéysd-) 🡺 'ghost'?
On October 31 2016, Kevin Stroud wrote
The connection between “ghost” and “guest/host” is mentioned on page 303 of ‘The Horse, The Wheel and Language” by David W. Anthony [quoted on English Stack Exchange]. As I noted in the early episodes of the podcast, that book was one of my primary sources for the Indo-European material. However, you are correct that most etymology sources suggest that “ghost” has a different PIE root. I would probably remove the reference to “ghost” in the “guest/host” discussion if I was preparing that episode today.
Undoubtedly, 'ghost' relates to "the notions of excitement, amazement, or fear" in *gheis-. Historically, vengeful spirits from ‘the other side’ have been an enormous source of fear, and ghost stories are scary stories — this connection is obvious.
Regarding the spelling, there is no single standard for how PIE is transcribed. Etymonline generally does a very bad job, ignoring all diacritics and thus conflating distinct phonemes; Wiktionary uses y and w for the semi-vowels that most transcription schemes would write as _i̯ _ and _u̯ _ or even just i and u. The palatal g is commonly either ǵ (as here), ĝ or g̑.
But how does 'ghost' semantically appertain to "*ǵʰéysd- (“anger, agitation”)"? How do ghosts anger, or are angered? And I've never heard humans being angered, or getting angry, at ghosts?