The term 'WHAT'S HAPPENIN'?' can be solved in a crossword puzzle by two different solutions: SUP (3 letters) and HOWGOESIT (9 letters). SUP is an informal greeting that stands for 'What's up?'. It is used as a way to ask someone how they are doing or what is going on in their life. HOWGOESIT is another informal greeting that is used to ask someone the same question. It is a combination of the words 'how' and 'goes it'. Both of these greetings are used as a way to ask someone 'What's happening?' or 'How are you?'.