Definition of "vacantly"

Mao gave no indication that he had heard or understood Jakob; his clouded eyes were fixed vacantly on one of the long windows of the old Ming dwelling beyond which the waters of Chung Nan Hai — the Central and South lakes — sparkled in the afternoon sunshine.

1988, Anthony Grey, Peking: A Novel of China's Revolution, 1921-1978‎, Little, Brown and Company, page 580