That's interesting, is that where wheest comes from?
Probably the other way around. Ist comes from wheest. Just like "is" is a shortened form of "agus"
In Irish we say "fuist" and it's anglicised as "whisht". Not sure how fuist lost its F sound in Gàidhlig and changed to W in English!
Does fuist ever get lenited? As fhuist could easily account for both ist and "whisht"?
Hmmm not to my knowledge at least. It's just an interjection, something your Mam might say to you when you were younger.
"Fuist! Éist liomsa!" (Whisht, listen to me!)