The Collaborative International Dictionary of English v.0.48:

Tail-bay \Tail"-bay`\, n.
   1. (Arch.) One of the joists which rest one end on the wall
      and the other on a girder; also, the space between a wall
      and the nearest girder of a floor. Cf. Case-bay.
      [1913 Webster]

   2. The part of a canal lock below the lower gates.
      [1913 Webster]