feather wedging
A wedge of hard wood or metal, to secure a tenon, pin, or the like, firmly in a mortise or other hole which does not extend through the piece, by its insertion part way into the end of the tenon or pin, so that, when driven into the hole, the wedge is forced up into the tenon or pin, forcing its sides apart. When the bottom of the mortise is wider than the mouth, a tenon so wedged cannot be withdrawn, as it then forms a dovetail. (Sturgis, 1900)
foxtail wedge