You can think of piss as one plain, rough word for the body letting liquid out, and for that liquid itself. Almost everything else the word does grows from two things in that picture. First, the liquid comes fast and freely and is hard to hold in. Second, it is the thing nobody wants. Rain that pours down, laughter you cannot stop, a drink that is not worth drinking, a person you are putting under you — the same picture is doing the work each time. The word also carries its own roughness. Saying it is a small act of choosing not to be polite, which is why so many of these uses live in jokes, insults and complaints.
Wiktionary traces piss to Middle English pisse and pissen, from Old French pissier, possibly from Vulgar Latin *pissiare, and says the root is probably imitative — the word copies the sound of the liquid. It pushed out the older English words micge (urine) and migan (to urinate).
Wiktionary 記這個字來自中古英語 pisse／pissen，再往上是古法語 pissier，可能出自通俗拉丁語 *pīssiāre。它多半是個擬聲字，也就是照著水聲造出來的。它後來取代了古英語的 miċġe（尿）和 mīgan（撒尿）。名詞和動詞一開始就是一組，所以今天兩邊都有一整排用法。
Wiktionary entry for piss, supplied in this packet (source_ref kaikki:3f6f69a190036ae4)
Six branches
字面：排尿與尿液
這一群就是核心畫面本身：動作、一次動作、以及流出來的東西。其他所有用法都從這裡往外長。
verb · B1
To pass urine out of your body. It is a rude word for this, used mostly among close friends or when someone is angry, and it can take an object for what comes out with the urine, as in piss blood.
撒尿（粗俗）
像整桶倒下來：大雨與憋不住的笑
從核心畫面裡「液體來得又急又多」這一點延伸出去。跟身體已經沒關係了，只剩下那個一直往下倒、擋不住的感覺。
verb · B2
Used to say that it is raining very hard. British English and rude. The subject is always it, and with rain can be added or left out.
下大雨（粗俗）
最不值錢的東西：白費與赤貧
這一群把核心畫面當成一把量尺：尿是最不值錢、最沒用的東西，所以拿它來說「這件事沒有用」和「他什麼都沒有」。
verb · B2
To waste your time and effort on something that cannot work at all. The picture is of urinating into the wind, so everything blows straight back at you.
白費力氣
往別人身上潑：取笑、輾壓、趕人
這一群把核心畫面對準了人。往別人身上撒尿，是英語裡把人放到最低位置最粗的一個動作，所以延伸出來的意思都是「我壓在你上面」。
noun · B2
To make fun of a person or a thing by laughing at them or copying them so that they look stupid. British English, and rude.
取笑；虧人
酒
這一支是從一句嫌棄長出來的：先拿尿來罵酒難喝，罵久了 the piss 就直接變成酒的代稱。它是玩笑式的延伸，不是另一條字源。
noun · C1
Alcoholic drink, especially cheap drink that tastes bad. It is rude, and it says the drink is not worth drinking.
劣酒；難喝的酒
身體裡的一股火氣
這一支離核心比較遠：它借的不是尿本身，而是「身體裡有東西在衝」的感覺，再配上醋的嗆勁。
noun · C1
To be full of loud, restless energy — lively, noisy and ready to take something on or argue about it. American English and rude.
精力旺盛；生龍活虎