Same entry in the Synagoge (upsilon39) and Photius' Lexicon (upsilon73 Theodoridis). The headword -- otherwise unattested but evidently quoted from somewhere -- is present indicative, third person singular, of the intransitive verb u(pani/sxw.
This verb can refer to the slow the rising of the sun or the moon, but perhaps, if the gloss is accurate, does not do so here.