正如樓上所說,孫楠唱得沒錯.
不見不散：Be there or be square.
A 'square' is a person who is ordinary or conservative in behavior or demeanor,not 'cool' or 'in.' The writer is saying that if you are not part of the scene or the action,then you are not cool.I think this is relatively new in usage,but I am unsure of the origin.
Immediately after World War II the word "square," which had long meant honest,direct,solid,became the antonym of hip or hep or sphisticated,at least in the U.S.A.Someone who didn't like or pretend to understand jazz and swing was "square." The word came to have a somewhat broadened use as out of touch,conventional,old-fashioned,not "with it." The latter expression is as dated as "Be there or be square."