The headword (presumably extracted from the first quotation given) is the aorist active participle, masculine nominative singular, of dusqete/w. In fact this verb occurs almost exclusively in participle forms in all periods (mainly in historians such as Xenophon, Polybius, and Diodorus Siculus; it is rare in other writers).
There is little substantive difference between its active and middle voices, both illustrated here.
[1] Polybius fr. 128 Büttner-Wobst. For Nabis see generally nu 5. Büttner-Wobst notes (p. 531) that Ursinus (Fulvio Orsini, 1529-1600) attributed the fragment to Polybius. However, noting that there is no instance of dusqetei=n in the active voice elsewhere in Polybius, Schweighäuser remained doubtful that the fragment was genuinely Polybian (Walbank, p.752). Walbank (ibid.) suggests that the fragment could be correlated with Nabis's (202-201 BCE) plans to attack Messene (cf. mu 659 and Polybius 16.13.3 at web address 1).
[2] This first gloss is also in ps.-Zonaras s.v.
[3] Polybius 33.17.1 (participle only) and 2 (quoted phrase, with a slight difference: the original is dusqetou/menoi toi=s sumbai/nousin a)poduspetei=n a)nagka/zontai, which occurs in alpha 3306 without any difference). The direct source is Constantinus VII Porphyrogenitus, De sententiis 200.27.