Please read Alexey Sklyarenko's annotations on Pale Fire, Ada and other Nabokov works here.
In his apologetic note to Lucette (in VN's novel Ada, 1969, Van's and Ada's half-sister) written after the dinner in ‘Ursus’ and debauch à trois in Van’s Manhattan flat Van says "we apollo" (meaning that he and Ada apologize for coaxing Lucette into their lovemaking):
Van walked over to a monastic lectern that he had acquired for writing in the vertical position of vertebrate thought and wrote what follows:
Poor L.