Lewis Short
sŭpīnĭtas (noun F) : supinus
* A bending backwards (very rare).
* Lit.: est odiosa omnis supinitas,Quint. 11, 3, 122.
* Transf.: Planasia (insula) de supinitate sic vocata,its lying flat, flatness,Sol. 3, 2.
Charlton T. Lewis, Charles Short, A Latin Dictionary