The word "mirthest" is an older form of the verb "mirth," which means to experience or express joy or laughter. It's not a commonly used word today, and it's important to note that it's spelled with the suffix "-est" at the end, indicating the superlative form of the verb. In IPA phonetic transcription, "mirthest" would be spelled /ˈmɜrθɪst/, with the stressed syllable being the second one, and the "th" sound being pronounced as /θ/.
The word "mirthest" is derived from the Middle English term "mirthe", which itself comes from the Old English word "myrgth". "Myrgth" is related to the Old Norse word "mirth", which means joy or pleasure. The addition of the "-est" suffix, used to form the superlative degree in English, gives the word "mirthest" the meaning of "most joyful" or "happiest". However, it is important to note that "mirthest" is an archaic form, no longer in common usage in modern English.