The correct spelling of the term "most subzero" relies on understanding its pronunciation. Using the International Phonetic Alphabet (IPA), we can break down the pronunciation into four sounds: /məʊst/ /sʌbzɪərəʊ/. The first sound, /məʊst/, describes the superlative form of "most," meaning it conveys the highest degree of something. The second sound, /sʌbzɪərəʊ/, combines the prefix "sub" meaning under or below and the word "zero." Together, the word "most subzero" communicates the strongest level of coldness imaginable.
"Most subzero" is not a standard English word or phrase, and therefore does not have an etymology. "Subzero" by itself typically refers to temperatures below zero degrees Celsius or zero degrees Fahrenheit. However, if you meant to ask about the etymology of the word "subzero", it originated as a combination of the prefix "sub-" meaning "below" and the word "zero" referring to the numerical concept of nothing. The meaning of "subzero" relates to temperatures below freezing point.