Where does “sībilāre” come from?

sībilāre (Latin) comes from Latin sībilus, from Proto-Indo-European sweysd- — to hiss.

Ancestry of “sībilāre”, step by step

Step	Language	Word	Meaning
1	Latin	sībilus	—
2	Proto-Indo-European	sweysd-	to hiss

sībilāre — Etymology, Origin & Word History | Etymology Explorer