Lewis Short
(adverb) : subtus, from sub; like intus, from in
* Below, beneath, underneath (mostly ante-class.; not in Cic. or Caes.): subtus ambulare,Cato, R. R. 48, 2: labra subtus pendula,Varr. R. R. 2, 9, 3: Romani aggere et vineis et omnibus supra terram operibus, subtus Macedones cuniculis oppugnabant,Liv. 36, 25; cf. Plaut. Cas. 5, 2, 26; Varr. L. L. 5, § 131 Müll.
Charlton T. Lewis, Charles Short, A Latin Dictionary