Where does “tẽero” come from?

tẽero (Old Portuguese) comes from Latin tener, from Proto-Indo-European ten- — to stretch, to extend.

Ancestry of “tẽero”, step by step

Step	Language	Word	Meaning
1	Latin	tener	soft, delicate, tender; young, youthful;...
2	Proto-Indo-European	ten-	to stretch, to extend

tẽero — Etymology, Origin & Word History | Etymology Explorer