Where does “สุมาตรา” come from?

สุมาตรา (Thai) comes from Indonesian Sumatra, from Malay Sumatera, from Sanskrit समुद्र, from Sanskrit सम्-, from Proto-Indo-Iranian sam-, from Proto-Indo-European sem- — together, one.

Ancestry of “สุมาตรา”, step by step

Step	Language	Word	Meaning
1	Indonesian	Sumatra	—
2	Malay	Sumatera	—
3	Sanskrit	समुद्र	the sea; ocean
4	Sanskrit	सम्-	with, together with, along with, together,...
5	Proto-Indo-Iranian	sam-	together, joint, paired
6	Proto-Indo-European	sem-	together, one

Every word from Proto-Indo-European sem- →Every word from Sanskrit सम्- →Every word from Sanskrit समुद्र →