Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
summussi = murmuratores (murmurers). Naevius: odi, inquit, summussos; proinde aperte dice, quid sit, Fest. pp. 298 and 299 Müll. (Trag. Rel. p. 12 Rib.) [submusso].