A thiol is basically an alcohol, but instead of an -OH group, they have a -SH group. That's where the "ol" part comes from.
The "thi-" part comes from "theion," the Greek word for sulfur. It's also an ancient word for divine fire, e.g. the "fire" part of "fire and brimstone." Very interested in this sulfur / bad smell / satan cloud of meanings around all of it.