Yes and no. If one had very low skin resistance, and put one hand on the negative terminal and the other on the positive terminal, the DC *current* thru the heart could be high enough to cause death. There was one case of this happening about 45 ears ago when such a person got across a 6V car battery. Using the tongue, there is very low resistance, and one could burn it and cause bleeding; it is possible that trauma could do the rest.