Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
tē̆trachmum, v. tetradrachmum.
† tē̆trădrachmum and tē̆trach-mum, i, n., = τετράδραχμον, a silver coin of four drachmas among the Greeks, Cass. ap. Cic. Fam. 12, 13, 4.
Form tetrachmum, Liv 34, 52, 6; 37, 58, 4; 37, 46, 3; 39, 7, 1 Weissenb.