The headword, evidently quoted from somewhere but attested only here, is genitive singular of the masculine noun po/sqwn.
The nominative -- from Aristophanes, Peace 1300 (web address 1) -- occurs with gloss in Hesychius pi3097 and Photius pi1114 Theodoridis: they say it is a colloquial word for a boy, from po/sqh (pi 2113 and note).
In Lucian, Lexiphanes 12, the word refers to a man with a big po/sqh (web address 2).