The silliest damn musical you ever saw is "Nightmare!!" the New Playwrights' Theatre's Christmas offering, now back as an out-of-season treat.

To say that it is good-natured fun is to do it an injustice, because it is anything but sappy. But the wit of Tim Grundmann, who wrote the book music and lyrics, and of the cast of top-flight comedians directed by Ken Bloom, has the race ability to be devastating without being condescending. It is in the bset tradition of early movie comedy - outrageous but sympathetic, satirical but humanistic.

The characters sre from American popular culture two clean teens, Henry and Bambi, olayed by Gardner Hathway and Debra Cerruti; Tanis Roach as an infinitely resourceful Italian mother who turns out her own fish sticks and piano concerts; and Chris Kauffman as the quintessentially helpful Irish parish priest, gamely taking a total-immersion course in exorcism. We follow them through good times and bad, in the parish of St. Jo-Ann's, while the play steers a precarious course between the crazy and the tasteless, occasionally falling down on one side or the other but, like Henry on his bicycle and Bambi on her roller skates, always managing to pick up and go bouncing.

If any of these roles had been written or played with the slightest him that they were contemptible or hypocritical, the paly wouldn't have worked. The skill was to refrain from spicing it up with malice or dumping into it the sugar of nostalgia.