The spelling of the word "most diuturnal" can be quite tricky due to its uncommon usage. It is pronounced as /moʊst daɪjʊˈtɜrnəl/. The word means "most active during the day", and is derived from the Latin word "diurnus", which means day. The phonetic transcription of this word helps to break down its complicated pronunciation, with the stress on the di- prefix. This word is an interesting addition to one's vocabulary, as it is not commonly used but adds a certain flair to one's lexicon.
The word "diuturnal" itself is quite rare and is derived from the Latin word "diuturnus", which means "lasting a long time" or "enduring". It is a combination of "diu" meaning "for a long time" and "turnus" meaning "a turning or rotation".
However, the phrase "most diuturnal" appears to be a combination of the superlative form "most" and the adjective "diuturnal". In this context, "most" intensifies the meaning of "diuturnal", emphasizing the quality of something being very long-lasting or enduring.
Please note that "diuturnal" is not commonly used and may be found in specialized contexts, such as technical or scientific literature.