the beginning of the Sun's shining (sinuti = to dawn). Also, possibly month of begging, from prositi = beg.
Some names are derived from archaic Croatian-Slavic words that are no longer found in standard Croatian dictionaries. In some cases even the meaning of those words is ambiguous.
In Macedonian, Listopad is also the (archaic) name for October but in Polish, Czech listopad, Belarusian 'listapad' ('Лiстапад') and Ukrainian "листопад" means November. Likewise, Polish sierpień, Czech srpen and Ukrainian "Серпень" mean August rather than July, and Polish lipiec, Belarusian Лiпень, and Ukrainian Липень' means July rather than June.