Piss down
Definition 1 of 1
Pro Tip 1/3Informal Usage
Used mainly in casual conversations among friends or in informal settings, not in professional or formal contexts. 
The festival was fun until it began to piss down and everyone got soaked.
Pro Tip 2/3Colloquial UK
Primarily used in the UK, so English learners using this phrase in the US or other countries might not be understood. 
Remember in the UK, 'piss down' is common, but in the US, they might say 'rain hard' instead.
Pro Tip 3/3Strong Emotion
Conveys frustration or annoyance, especially if the heavy rain disrupts plans. 
We were all set for the match, but it pissed down and ruined it.