E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
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. (Duncan: Russia.)