The term 'DO SUM-THING' is a play on words, combining the phrase 'do something' with the mathematical term 'sum-thing, ' which is a shorthand way of saying 'the sum of something.' The crossword solutions for this term are 'ADD, ' which is a three-letter word meaning to join two or more things together. This term can be used in many contexts, such as when someone wants to encourage another person to take action and make a difference.