Tired like ham

There is a Canto saying.. 'Kui tou ham kum yeung' .. Which literally translates to 'tired like ham'...  But if u look at pictures of ham, there doesn't seem to be any association between being tired and ham. Looking at pics of hawk from nanatsu no taizai, it seems like everyone around him looks tired... And he seems more than awake most of the time. So where does that ham come from? There is another word people use which is a bad word, but we shouldn't go there.
I'm sooo tired I'm even starting to type rots...  Sigggghhhh..