Be chucking (it) down American British spoken
To rain very heavily
We scheduled some outdoor activities but everything is postponed because it's chucking down.
It's chucking it down outside, so we can't go hiking this morning.
Jane encountered a host of difficulties driving the car on the way back home while it was chucking it down.
If the heavens open, it starts to rain heavily.
A heavy rain
A unusually severe or torrential rainfall of rain.
Used to describe a heavy rain or a downpour
To rain heavily and at great length
The verb "be" in this case is only used in the singular form.