The word "grindest" is a variant spelling of the word "grinder," which refers to a machine used for grinding. In IPA phonetic transcription, "grindest" is pronounced as /ˈɡraɪndɛst/. This pronunciation involves stressing the first syllable of the word, followed by a long "i" sound in the second syllable. The "d" on the end of the word is pronounced like a voiced /t/ sound due to the ESD rule in English pronunciation, which states that voiceless consonants are pronounced as voiced consonants when they come before a voiced sound.
The word "grindest" does not have a separate etymology distinct from the base word "grind". "Grind" is derived from the Old English word "grindan", which means "to crush into small pieces by rubbing together". The word has remained relatively unchanged over the centuries and is still used today in the same context. However, "grindest" is not a commonly used form and does not have a specific etymology of its own.