Green’s Dictionary of Slang

(US) a sidewalk or pavement grating.

1951

(con. 1910s) Green & Laurie Show Biz from Vaude to Video 10: Alf Grant gave a classic example of the slang in vogue among the show people of that day when asked if he had seen a certain actor. ‘Sure,’ he replied. ‘I saw him standing on the Hope Island (in front of the Palace) on the waffle iron (subway grating on sidewalk) about ten minutes ago.’.