Lewis & Short

sŭpīnĭtas, ātis, f. [supinus], a bending backwards (very rare).

  1. I. Lit.: est odiosa omnis supinitas, Quint. 11, 3, 122.
  2. II. Transf.: Planasia (insula) de supinitate sic vocata, its lying flat, flatness, Sol. 3, 2.