The headword is an adjective in the masculine nominative singular; cf. kappa 1941 (gloss), lambda 860 (gloss), omega 275 (gloss), sigma 430 (gloss), sigma 436, and see generally LSJ s.v. The headword appears in the quotation given (n. 3 below), where the sense applies to sloped ground. This is incongruent with the gloss, which is a more plausible indication of the source of the entry, where the lemma applies to human noses: see next note.
[1] Both the headword and the opposed glossing adjective grupo/s, appear -- with the latter in the neuter accusative -- in Plato, Republic 474D (web address 1) in the course of Socrates’ comparison of the cuteness of snub-nosed boys versus the regal look of hook-nosed youths.
[2] For simos as snub-nosed see LSJ s.v., citing instances from Xenophanes and Herodotus onwards. But this phrase with 'in the nose' explicit is uncommon: its earliest extant attestation is in the Vita Aesopi (Accursiana), a C1 CE text compiled by Maximus Planudes (ca. 1260 – ca. 1305 CE); cf. OCD(4) s.v. anthology; Fisher in Kazhdan s.v. Planoudes; and Eberhard, pp. 226-305.
[3] Philostratus, Life of Apollonius of Tyana 2.11: Damis offers this advice (elicited by Apollonius upon their observation of an Indian boy roughly treating an elephant) about good horsemanship. For this sense of the headword see LSJ s.v., II.1, and sigma 436. [In her critical apparatus Adler reports that ms V transmits a)fei=nai, thus reading to let loose the bit.]