The term 'UGLY AS ___' can be completed with two crossword solutions: SIN (3 letters) and ATOAD (5 letters). The phrase 'ugly as SIN' is an idiom that has been used since the 16th century to describe something that is extremely unattractive. It is derived from the biblical concept of SIN being the source of all evil and ugliness. The phrase 'ugly as a toad' is a more modern version of the same idiom, which implies that toads are ugly creatures. Both phrases are used to describe something that is very unpleasant to look at.