The word "causticnesses" is spelled with three syllables: /kɔːstɪknəsɪz/. The first syllable "caus-" is pronounced with a "k" sound, followed by a long "awe" vowel sound, and ending with an "s" sound. The second syllable "tic" is pronounced with a short "ih" sound, and ending with a "k" sound. The third syllable "nesses" is pronounced with a long "eh" vowel sound, followed by a "z" sound, again emphasizing the "s" and "z" sounds. Overall, the phonetic transcription explains the spelling of this word, which refers to the quality of being corrosive or burning.
The word "causticnesses" does not have a commonly accepted etymology because it is not a widely recognized word. It appears to be a plural form derived from the adjective "caustic". "Caustic" originated in the late 14th century from the Latin word "causticus", which means "burning" or "corrosive". This Latin term was in turn derived from the Greek word "kaustikos", meaning "capable of burning". However, the addition of the "-nesses" suffix to create "causticnesses" seems to be an uncommon or potentially nonstandard formation.