I believe that any respectable dictionary must include as one of the definitions of “sui generis” the words “Nellie McKay”. Has anyone encountered anywhere another such being, so precocious, eccentric, old-timey, modern, sweet, caustic, irreverent, generous, unforgiving, feminine, feminist, brilliant, faux naïve, and Nellie-knows-how-many-other qualities that I can’t begin to put my finger on? Is she a singer-songwriter, a chanteuse, a piano player, a satirist, a comedienne, a polemicist for good-hearted causes, a musical comedy sensation? Alas, the words fall short and the categories diminish her.

The producers of OLD HATS at the Signature Theater have somewhere quoted a reviewer who averred that the show’s stars, Bill Irwin and David Shiner, would have had to invent her did she not exist already. That is true enough, but also an unwitting admission that had this been simply two hours with Nellie McKay, she would have had no need to summon up an Irwin and a Shiner, whose particular brand of New Vaudeville feels a bit, well, old hat next to her smart and dizzy artistry. OLD HATS is really just an old fashioned revue, and despite the use of cellphones and the like in some of the routines, it is McKay who makes it feel relevant and contemporary.

She is the only one of the three who appears to take the world seriously, which everyone from Shakespeare to Fellini has recognized as the essence of great clowning. Some of the routines in OLD HATS are very funny, especially the magic show and cowboy movie spoofs (which enlist audience members as performers). But almost everything that Irwin and Shiner do comes off as just for yuks. Even the overt political satire (a debate between two bucktoothed candidates) and the jabs at capitalism (a rubbery limbed businessman absorbed in his deal making) are little more than generic gags. Unlike the average person today, all the characters in the show wear hats, old ones. It is true that wearing different hats at different times has survived in our vernacular, but haberdashery isn’t the marker of identity it was when the expression came into being: the show’s basic concept militates against its currency.

It is McKay’s musical acumen and lyrical brilliance (she is as verbally sharp as a Porter, a Coward, or a W.S. Gilbert) that truly bridges the gap between old forms and new themes. I went to, saw, and enjoyed OLD HATS (especially in its second half), but as much as anything it made me want to see McKay the next chance I get, at the center of her own show, surrounded as she usually is by talented friends, but with no need to have invented any of them.

For information on upcoming shows at the Signature Theatre, click here. OLD HATS returns Jan. 26-March 6, 2016, featuring Shaina Taub in lieu of Nellie McKay.