The term 'COOL, ONCE' can be completed with the following crossword solutions: GROOVY (6 letters), RAD (3 letters), PHAT (4 letters), BOSS (4 letters), NEATO (5 letters), and HEP (3 letters). These words are all slang terms that have been used to describe something that is fashionable, attractive, or impressive. GROOVY is a term that was popularized in the 1960s and 70s and is still used today to describe something that is cool or hip. Rad is an abbreviation of the word radical, which is used to describe something that is exciting or extreme. PHAT is another term that has been around since the 1980s and is used to describe something that is attractive or desirable. BOSS is a term that is used to describe something that is excellent or outstanding. Neato is a term that is used to describe something that is neat or orderly. Finally, HEP is a term that was popularized in the 1940s and is used to describe something that is fashionable or up-to-date.