"Mother yaw" is a combination of two words – "mother" and "yaw". The word "mother" is written as /ˈmʌðər/ in IPA, where the symbol /θ/ represents the "th" sound. "Yaw" is written as /jɔː/ in IPA, where the symbol /j/ represents the "y" sound. Therefore, the correct spelling of "mother yaw" should be "mother yawn", which means to open one's mouth wide and inhale deeply due to tiredness or boredom. Always be careful with spellings, as they can change the meaning of a word entirely.
A notably large tubercle in the yaws eruption.
A practical medical dictionary. By Stedman, Thomas Lathrop. Published 1920.