You can think of vulgarity as what shows when the cover that polite life keeps over things slips off: the joke nobody asked for, the laugh at the wrong moment, the gold on everything. The plain word names that quality in the air; the plural names the actual items that fall out of it.
From vulgar + -ity, from Latin vulgaritas. The suffix -ity turns an adjective into the name of its quality, which is exactly the split you see on the page: vulgar describes, vulgarity names.
由 vulgar 加上 -ity 構成，來源是拉丁文 vulgaritas。-ity 的功能是把形容詞變成「那個性質的名字」。這正好對上這一頁的結構：vulgar 是形容，vulgarity 是把那個形容變成一個可以談論的東西。
Wiktionary etymology supplied in the evidence packet (kaikki:3f6f69a190036ae4:1144130263): From vulgar + -ity, from Latin vulgaritas.
One branch
粗俗這件事，以及掉出來的那些話
兩個意思是同一件事的兩個角度：一個是那股勁兒，一個是那股勁兒說出口的樣子。文法上的差別就是不可數對複數。
noun · C1
The quality of being vulgar: either saying and doing things that are rude and offensive, or showing a liking for things that look showy and cheap rather than in good taste.
粗俗；庸俗