Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
sub-tĕro, trīvi, trītum, 3, v. a., to rub off or wear away underneath; to rub, bruise, or grind to pieces (rare; not in Cic.): boves ne pedes subterant, Cato, R. R. 72; so, pedes, Col. 6, 15, 2; Plin. 28, 16, 62, § 221: ungulas, Plaut. As. 2, 2, 73: jumenta cito subteruntur, Sen. Ep. 51, 9: salis sextarium subterito, et subtritum, etc., pound, triturate, Col. 12, 5, 1: caepam aridam, id. 12, 5, 56; 2, 10, 25; Plin. 20, 20, 82, § 218; 30, 9, 23, § 80.