Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
sublestus, a, um, adj. [acc. to Döderl. Synon. 2, p. 101 sq., perh. from sublevo; and hence, light], slight, trifling, trivial (ante-class.): sublesta antiqui dicebant infirma et tenuia, Plaut. Pers. 3, 1, 20: fides sublestior, id est, infirmior. Idem in Nervolaria vinum ait sublestissimum, quia infirmos faciat vel corpore vel animo, Fest. pp. 294 and 295 Müll.; cf.: sublestum est leve, frivolum, Non. 177, 11: sublestus infirmus, tristis, Gloss. Isid.: fides, Plaut. Bacch. 3, 6, 13.