Green’s Dictionary of Slang

a still-born child.

1864, 1867, 1870		Hotten Sl. Dict. 247: stills the undertakers’ slang term for still-born children. The fee paid by nurses and others is usually 2s. 6d. A separate coffin is never given; the stills are quietly introduced into one containing an adult about to be buried. stills are allowed to accumulate at an undertaker’s until they sometimes number as many as a dozen.
1890–1904		Farmer & Henley Sl. and Its Analogues.
1897		W.S. Maugham Liza of Lambeth (1966) 95: I’ve ’ad twelve, ter sy nothin’ of two stills an’ one miss.