"structure consisting of one large, unhewn stone slab resting on two or more stones placed erect in the earth," such that a person may walk under it, 1859, from French dolmin applied 1796 by French general and antiquarian Théophile Malo Corret de La Tour d'Auvergne (1743-1800), perhaps from Cornish tolmen, literally "hole of stone," from Celtic men "stone."
Some suggest the first element may be Breton taol "table," a loan-word from Latin tabula "board, plank," but OED says the Breton form of this compound would be taolvean. "There is reason to think that this [tolmen] is the word inexactly reproduced by Latour d'Auvergne as dolmin, and misapplied by him and succeeding French archaeologists to the cromlech" [OED]. See cromlech, which is the same idea but arranged as one of a circle. Related: Dolmenic.