Where does “свѣи” come from?

свѣи (Old East Slavic) comes from English swear, from Middle English sware, from Old English swaru, from Proto-Germanic swarō, from Proto-Indo-European swer- — to ache; to fester; wound; injury.

Ancestry of “свѣи”, step by step

свѣи traces back through two separate lines of ancestry.

via English swear

StepLanguageWordMeaning
1EnglishswearTo take an oath, to promise; To use offensive,...
2Middle EnglishswareAlternative form of square; A response to a query...
3Old Englishswaruan oath, vow
4Proto-Germanicswarōa solemn statement or affirmation; oath; vow
5Proto-Indo-Europeanswer-to ache; to fester; wound; injury

via Old Norse svíar

StepLanguageWordMeaning
1Old Norsesvíarthe Swedes of Sweden proper
2Proto-Germanic*swihaniz
3Proto-Germanicswihôa Swede
4Proto-Germanicswe-"one's own, self"
Every word from Proto-Indo-European swer-Every word from Proto-Germanic swarōEvery word from Old English swaru