Where does “Breatainn” come from?

Breatainn (Scottish Gaelic) comes from Old Irish Bretain, from Latin Brittōnēs.

Ancestry of “Breatainn”, step by step

Step	Language	Word	Meaning
1	Old Irish	Bretain	Britain
2	Latin	Brittōnēs	—

Breatainn — Etymology, Origin & Word History | Etymology Explorer