heavy rain; steady rain; downpour
Describes rain that has become steady and heavy, as opposed to light or intermittent rain. Can also refer to heavy snowfall, but rain is the primary association.
It started raining heavily in the afternoon.
I walked home in the steady downpour.
Compound of 本 (hon, 'real, main') and 降り (buri, 'rainfall, snowfall'). The prefix 本 indicates that the precipitation is now in earnest.