The Collaborative International Dictionary of English v.0.48:

Underslung \Un"der*slung`\, Underhung \Un"der*hung"\, a.
   Of an automobile body, suspended from the springs in such a
   manner that the frame of the chassis is below the axles, the
   object being to lower the center of gravity of the car.
   [Webster 1913 Suppl.]

The Collaborative International Dictionary of English v.0.48:

Underhung \Un`der*hung"\, a.
   1. (Carp.) Resting on a track at the bottom, instead of being
      suspended; -- said of a sliding door. --Forney.
      [1913 Webster]

   2. Having the lower jaw projecting. --T. Hughes.
      [1913 Webster]

WordNet (r) 3.0 (2006):

underhung
    adj 1: supported from below especially resting on a track
           instead of suspended from above; "underhung sliding
           doors"
    2: having a lower part projecting beyond the upper; "an
       underhung jaw"; "undershot bulldog" [syn: underhung,
       undershot, underslung]