Pronunciation: [mˈə͡ʊst ˈə͡ʊld sˈɔː] (IPA) 
The correct spelling of "most old saw" is actually "mouldy old saw". The word "mouldy" is spelled with a "u" instead of an "o" because it is the British English spelling. The word "saw" is spelled as expected. In IPA (International Phonetic Alphabet) transcription, the pronunciation of "mouldy" is /ˈməʊldi/ and the pronunciation of "saw" is /sɔː/. So the correct pronunciation of the phrase is /ˈməʊldi ˈɔːld sɔː/.
"Most old saw" is an idiomatic phrase primarily used in informal or colloquial settings. This expression refers to a well-worn or frequently repeated saying or proverb that has been known or used for a significant period of time. It is a way to emphasize the familiarity or common knowledge associated with a particular adage or saying.
The term "old saw" itself is sometimes used interchangeably with "old chestnut" or "hoary chestnut." It originates from the use of a saw (a cutting tool with teeth) as a metaphor for an often-repeated statement that is considered conventional wisdom or folk wisdom. It suggests that such saws have been so extensively utilized or referenced that they have become tiresome or clichéd.
When the phrase is modified with "most," it serves as an intensifier, emphasizing the heightened degree of antiquity or prevalence associated with the old saw being discussed. It suggests that the saying in question is not just commonly known or frequently used but is among the most well-known or often-referenced ones.
Overall, "most old saw" refers to an adage, proverb, or saying that has achieved a notable level of familiarity, being widely circulated or recognized due to its age, frequent usage, or widespread acceptance as a kernel of wisdom. Its repetitive nature can sometimes lead to it being perceived as trite or lacking originality.
