The spelling of the word "most underaverage" is pretty straightforward once you break it down phonetically. Firstly, "most" is pronounced as /məʊst/, rhyming with "post". Secondly, "underaverage" is pronounced as /ʌndərˈæv(ə)rɪdʒ/, with the stress on the second syllable "der". To break it down even further, "under" is pronounced as /ʌndər/ and "average" is pronounced as /ˈæv(ə)rɪdʒ/. Overall, the spelling is quite logical and easy to understand when taking into account the sounds of each individual syllable.