The headword is present middle participle, masculine accusative singular, of the verb u(pote/mnw (for which see also upsilon 401, upsilon 605, upsilon 607, upsilon 608, upsilon 609, and upsilon 610). It is presumably quoted from somewhere, perhaps Artemidorus 1.4.
Other lexica have the same entry with nominatives; references at Photius upsilon259, where Theodoridis notes examples of u(potemno/menos (in this sense) in Aeschines and John Chrystostom. See also e.g. Xenophon, Hellenica 1.6.15 (upsilon 607), and numerous post-classical instances.