horse-out

To cut (a piece of lumber) into a peculiar shape. In this sense, limited to one or two special members of a building; thus, a horsed string of a flight of stairs is one which has been sawed alternately horizontally and vertically so that the upper edge shows a series of right-angled projections to which may be secured the treads and risers which are to form the steps. The string of a stair so treated is said to be horsed or horsed-out, as distinguished from ploughed or ploughed-out; and the same terms are used for the completed stairs. (Sturgis, 1900)

horse