You can picture pissy as piss with -y stuck on the end: something that has urine on it, or smells of it. English often reaches for that wrinkle-your-nose smell when it wants to put someone or something down. Once you can smell the literal picture, the other two uses read as the same smell thrown in different directions — at a person's attitude, and at a thing that is not worth much.
可以想成：pissy 是 piss（尿）加上 -y。最原本的畫面就是字面的——沾到尿、聞起來一股尿騷味。英文在罵人罵事的時候，常常就借這種讓人皺眉的味道來出力。先聞到這個字面畫面，另外兩個用法就好記了：同一股味道，一邊潑在人的態度上，一邊潑在不值一提的東西上。
From the word’s history
The packet's source derives pissy simply from piss + -y, which is why the literal smell sits at the centre rather than at the edge of the word.
來源記到 pissy 就是 piss 加上形容詞字尾 -y。所以字面的尿味不是這個字的邊緣用法，而是它的中心；罵脾氣、罵東西都是後來從這裡延伸出去的。
Wiktionary, "pissy"
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