The headword as transmitted is nominative singular of the noun probolos; the quotation given below includes its accusative plural. For its range of literal and figurative applications, all stemming from its root meaning of anything that projects, see LSJ s.v. However, the Suda's probolos should surely have been proboulos: see next note.
[1] Likewise or similarly glossed in other lexica; references at Photius pi1199 s.v. pro/boulos (sic), which is surely correct for the glossing phrase given. For 'council-chamber' in a context like this, see generally LSJ s.v. bouleuth/rion, II.
[2] Eunapius fr.43 FHG (4.33); Blockley, Eunapius fr. 45.3. Commenting on this passage in FHG (4.33) Müller, Boissonade suggested that the passage describes the efforts of Valens (emperor 364-378 CE) to suppress forays by Scythian Huns into Thrace; cf. PLRE s.v. Flavius Valens 8, p. 930; omicron 764.