understrap

noun

1. A strap running underneath something.

   “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[…]”

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