Where does “tonnā” come from?

tonnā (Proto-Celtic) comes from Proto-Indo-European tend-, from Proto-Indo-European temh₂- — to stretch, string; to cut.

Ancestry of “tonnā”, step by step

Step	Language	Word	Meaning
1	Proto-Indo-European	tend-	to stretch, to extend; to split, cut off
2	Proto-Indo-European	temh₂-	to stretch, string; to cut