understrap

under- +‎ strap

understrap (plural understraps)

1. A strap running underneath something.
   • 1860, United States, Congressional serial set, Government Printing Office, page 127:

     In the chassis of the barbette carriage, the middle transom, instead of being held by bolts passing through it, is connected with the rails by understraps, which are fastened to the rails by one or two bolts on each side of the transom […]

   • 2002, Robin J. Harman, Patient care in community practice, page 40:

     The penis is inserted through a plate which is securely held to the pubis by a waistband and leg understraps (Figure 2.3).

• perdurants, underparts