The term 'HANDY DIGIT' is a combination of two crossword solutions, 'THUMB' and '5 letters'. The term 'HANDY DIGIT' refers to the THUMB, which is the most useful finger on the human hand. It is the only finger that has the ability to move independently from the other four fingers, making it the most versatile and useful digit. The '5 letters' part of the term refers to the fact that the word 'THUMB' is five letters long.