The phrase 'as old as the hills' is an idiom that means something is very old and has been around for a long time. The two crossword solutions, HOARY (5 letters) and ANCIENT (7 letters), both describe something that is very old. HOARY is an adjective that means gray or white with age, while ANCIENT is an adjective that means belonging to a distant past. Both words are used to describe something that has been around for a long time.