Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
tuxtax, an exclamation imitative of the sound of blows: tuxtax meo tergo erit, Plaut. Pers. 2, 3, 12; cf. Naev. ap. Charis. 2, p. 185 P. (Com. Fragm. v. 6 Rib.), tax pax.