"Fellest" is not a commonly used word in the English language. It is an adjective that means extreme, brutal, or savage. The antonyms for "fellest" can include mild, gentle, kind, and humane. The opposite of "fellest" can also be described as compassionate, empathetic, merciful, or lenient. The idea is to use words that evoke a sense of kindness, compassion, and gentleness instead of words that depict extreme violence or brutality. In everyday language, it is more common to use simpler words to describe the opposite of "fellest" rather than using a rarely used term.