Uncertain. Morphologically possibly from pro- (“through”) + sȋnjī (“gray; of the colour of ash”). Etymology of Christian source describes it as a month of "praying" (compare pròsiti (“to pray”)). Other sources speculate that, since it's the month of Sun's solstice, it would be the month in which the Sun "starts glowing" (compare prosínuti (“to start glowing”)).
pròsinac m (Cyrillic spelling про̀синац)
- (Croatia) December
u prosincu ― in December
dvadeset petog prosinca ― on the twenty-fifth of December