The headword, presumably extracted from the quotation given, is the aorist active participle of u(perisqmi/zw (untranslatable into English as a single word), masculine nominative singular. For a similar compound verb, see delta 1047 and delta 1048.
[1] Polybius fr. 162 Büttner-Wobst (quoted already at eta 342; cf. scholarly commentary on this fragment in the notes thereto). The keles and the hemiolia are types of ships. The isthmus in question is likely to be either the isthmus of Corinth (Barrington Atlas map 58 grid D2) or the one connecting the island of Leukas (Leucas, Barrington Atlas map 54 grid C4) to the NW Greek mainland.