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.]

WordNet (r) 3.0 (2006):

underslung
    adj 1: supported from above especially in a vehicle having
           springs attached to the axle from below
    2: having a lower part projecting beyond the upper; "an
       underhung jaw"; "undershot bulldog" [syn: underhung,
       undershot, underslung]
    3: having a low center of gravity; built low to the ground [syn:
       squat, underslung]