(in French, Dolmen). An immense mass of stone placed on two or more vertical ones, so as to admit a passage between them. (Celtic, tol or dol, table; men, stone.)
The Constantine Tolmen, Cornwall, consists of a vast stone 33 feet long, 14 1/2 deep, and 18 1/2 across. This stone is calculated to weigh 750 tons, and is poised on the points of two natural rocks.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894