So called from the word greben (a comb).
This title was conferred upon them by Czar Ivan I., because, in his
campaign against the Tartars of the Caucasus, they scaled a mountain
fortified with sharp spurs, sloping down from its summit, and
projecting horizontally, like a comb (
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894