J

n. (Arch) The vertical side of any opening, as a door or fireplace; hence, less properly, any narrow vertical surface of wall, as the of a chimney-breast or of a pier, as distinguished from its face. The vertical lining of an opening, such as a doorway, fireplace, window, etc. (Mining) Any thick mass of rock which prevents miners from following the lode or vein.

 - vb. See Jam, vb.