pissen
to piss
Colloquial, vulgar for to urinate.
Ich muss mal kurz pissen gehen.
I need to go piss real quick.

to piss
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to piss
Colloquial, vulgar for to urinate.
Ich muss mal kurz pissen gehen.
I need to go piss real quick.

The word 'pissen' (to piss) is very colloquial and is perceived as vulgar or crude. Use it only in very informal situations, for example among close friends.
Er fluchte und sagte, er müsse dringend pissen.
He swore and said he urgently needed to piss.
You can tell whether 'pissen' (to piss) means to urinate or to rain from the subject. If a person is the subject, it is about urinating. If 'es' (it) is the subject, it is about heavy rain.
Warte kurz, der Hund muss mal pissen.
Wait a moment, the dog needs to piss.
When 'pissen' (to piss down) means heavy rain, it is always used impersonally with 'es' (it) as the subject. So the structure is always: 'Es pisst.' (It's pissing down.)
Zieh deine Regenjacke an, es pisst schon wieder.
Put on your rain jacket, it's pissing down again.
In the meaning 'urinieren' (to urinate), the verb is often used in fixed phrases such as 'pissen gehen' (to go piss) or 'pissen müssen' (to need to piss).
Wir sollten eine Pause machen, ich muss mal pissen.
We should take a break, I need to piss.
to rain
'regnen' and 'pissen' can both talk about rain. 'regnen' is the normal and neutral word. 'pissen' is crude and vulgar; with weather, it means that it is raining heavily. Learners may confuse the words because in casual conversations you sometimes hear 'es pisst' (it's pissing down). But this is not polite and does not fit formal situations. 'es regnet' (it's raining) is almost always the safe choice; 'es pisst' is something you would say at most among friends and with a rough tone.

Nach dem Training pisste es so stark, dass niemand zum Bus laufen wollte.
After training, it was pissing down so hard that nobody wanted to run to the bus.
Wir bleiben im Zug sitzen, weil es draußen regnet.
We stay seated on the train because it is raining outside.