A swainmote seems in Medieval Britain to have been a sort of tri-annual assembly of the free men living within the bounds of a Royal Forest. I'm not sure how much solid information we have about these assemblies, but the etymology is interesting.
The first element, swain-, is now a rare word meaning a young lover, e.g.
'Who is Silvia? What is she, That all our swains commend her?'
Shakespeare, The Two Gentlemen of Verona
but an earlier meaning was 'country worker', 'young lad', 'servant', 'assistant'. The assistant meaning is seen in e.g. boatswain. Its ultimate origin is an ancient root PIE meaning 'oneself', which developed into the sense of something like 'my assistant'. It has many cognates and is seen in the given name Sven, a fairly common Scandinavian name, and seen it's believed in Swansea, 'Sven's Island' in old Norse, suggesting the settlement was founded by a Viking.
The second element -mote is seen in moothall, a place for community discussions, and a moot point, something debatable. It has cognates in pretty well all Germanic languages.
Swainmote is very rare in modern written English, at 0.01 occurrences per million words. So just by writing this blog post I've used it the number of times we'd expect in three hundred million words.