Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
1. tau, n. indecl., a Celtic word of unknown meaning, Verg. Cat. ap. Quint. 8, 3, 28; cf. Aus. Idyll. 12, 6.
2. tau, n. indecl., the name of the Greek T, Aus. Idyll. Lit. Monos. 18: et signa tau super frontes virorum gementium, Vulg. Ezech. 9, 4 (al. thau).