As far as I or Google can tell, “the Buddha is not serious” is my own phrase. Though a respectable explanation would entail a bit of Eastern philosophy, I think I can summarize it roughly like this: “The Buddha is not serious” means that clear-mindedness (whatever that means) necessarily contains elements of humor, spontaneity, and lightheartedness.
Alan Watts said something along the same lines, that “humor and self-righteousness are mutually exclusive.” That gives me an excuse to quote him at length, although the context is likely to be insufficient. Bolding mine.
No one can be moral—that is, no one can harmonize contained conflicts—without coming to a working arrangement between the angel in himself and the devil in himself, between his rose above and his manure below. The two forces or tendencies are mutually interdependent, and the game is a working game just so long as the angel is winning, but does not win, and the devil is losing, but is never lost. (The game doesn’t work in reverse, just as the ocean doesn’t work with wave-crests down and troughs up.)
It is most important that this be understood by those concerned with civil rights, international peace, and the restraint of nuclear weapons. These are most undoubtedly causes to be backed with full vigor, but never in a spirit which fails to honor the opposition, or which regards it as entirely evil or insane. It is not without reason that the formal rules of boxing, judo, fencing, and even dueling require that the combatants salute each other before the engagement. In any foreseeable future there are going to be thousands and thousands of people who detest and abominate Negroes, communists, Russians, Chinese, Jews, Catholics, beatniks, homosexuals, and “dope-fiends.” These hatreds are not going to be healed, but only inflamed, by insulting those who feel them, and the abusive labels with which we plaster them—squares, fascists, rightists, know-nothings—may well become the proud badges and symbols around which they will rally and consolidate themselves. Nor will it do to confront the opposition in public with polite and non-violent sit-ins and demonstrations, while boosting our collective ego by insulting them in private. If we want justice for minorities and cooled wars with our natural enemies, whether human or non-human, we must first come to terms with the minority and the enemy in ourselves and in our own hearts, for the rascal is there as much as anywhere in the “external” world—especially when you realize that the world outside your skin is as much yourself as the world inside. For want of this awareness, no one can be more belligerent than a pacifist on the rampage, or more militantly nationalistic than an anti-imperialist.
You may, indeed, argue that this is asking too much. You may resort to the old alibi that the task of “changing human nature” is too arduous and too slow, and that what we need is immediate and massive action. Obviously, it takes discipline to make any radical change in one’s own behavior patterns, and psychotherapy can drag on for years and years. But this is not my suggestion. Does it really take any considerable time or effort just to understand that you depend on enemies and outsiders to define yourself, and that without some opposition you would be lost? To see this is to acquire, almost instantly, the virtue of humor, and humor and self-righteousness are mutually exclusive. Humor is the twinkle in the eye of a just judge, who knows that he is also the felon in the dock. How could he be sitting there in stately judgment, being addressed as “Your Honor” or “Mi Lud,” without those poor bastards being dragged before him day after day? It does not undermine his work and his function to recognize this. He plays the role of judge all the better for realizing that on the next turn of the Wheel of Fortune he may be the accused, and that if all the truth were known, he would be standing there now.
If this is cynicism, it is at least loving cynicism—an attitude and an atmosphere that cools off human conflicts more effectively than any amount of physical or moral violence. For it recognizes that the real goodness of human nature is its peculiar balance of love and selfishness, reason and passion, spirituality and sensuality, mysticism and materialism, in which the positive pole has always a slight edge over the negative. (Were it otherwise, and the two were equally balanced, life would come to a total stalemate and standstill.) Thus when the two poles, good and bad, forget their interdependence and try to obliterate each other, man becomes subhuman—the implacable crusader or the cold, sadistic thug. It is not for man to be either an angel or a devil, and the would-be angels should realize that, as their ambition succeeds, they evoke hordes of devils to keep the balance. This was the lesson of Prohibition, as of all other attempts to enforce purely angelic behavior, or to pluck out evil root and branch.
–Alan Watts, The Book: On the Taboo Against Knowing Who You Are