Where does “цвијет” come from?

цвијет (Serbo-Croatian) comes from Proto-Slavic květъ, from Proto-Indo-European ḱwoyt-o-, from Proto-Indo-European ḱweyt- — to shine.

Ancestry of “цвијет”, step by step

Step	Language	Word	Meaning
1	Proto-Slavic	květъ	bright color, light; color, bloom, flower
2	Proto-Indo-European	ḱwoyt-o-	white, light
3	Proto-Indo-European	ḱweyt-	to shine

цвијет — Etymology, Origin & Word History | Etymology Explorer