Lewis & Short

subtus, adv. [from sub; like intus, from in], below, beneath, underneath (mostly ante-class.; not in Cic. or Cæs.): 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.