The word "hissest" is an uncommon variation of the word "hissed." It is pronounced /ˈhɪst/ with a short "i" sound in the first syllable and a soft "s" at the end. The reason for the extra "s" in "hissest" is likely due to an effort to emphasize the strong "s" sound when pronouncing the past tense of the verb "hiss." However, since this spelling is not widely recognized or accepted, it is best to stick with the standard spelling of "hissed."
The word "hissest" is the second person singular present form of the verb "hiss", which means to make a sharp sibilant sound, like that of a snake. The etymology of "hiss" can be traced back to the Middle English word "hissen", which in turn derived from the Old English word "hysan". This Old English word is believed to have originated from the imitative Proto-Germanic word "hissijaną", which imitated the sound of a snake hissing.