Curlers pass on more false origins (folk etymologies) about this term than you can shake a curling iron at. A friend of mine in Winnipeg once solemnly assured me it came from an immigrant who spoke German and French who called out, "Bon spiel!" at a game. Good play, indeed.
Sports records declare the first curling bonspiel in Canada took place in 1839 on a frozen Don River in Toronto.
A bonspiel is a match or tournament between curling clubs consisting of a competition comprised of a number of different events often played over a weekend. In Scotland, and increasingly in Canada, the word is shortened by curlers to spiel.
A Dutch term bond spel ‘league game’ has been in print since 1772. In Middle Dutch a bont was a group of people bound (bonded) into a league. Or, as a source, you may side with bonespel, a West Flemish children's game. But bonspiel first appears in print in Scotland during the middle of the 16 century. So the origin may lie hidden, forever overgrown in the heathery waysides of Scottish linguistic history.
But there is no doubt of bonspiel’s Germanic word relatives. The spel root is perhaps more familiar to English speakers in its German form of das Spiel ‘the game’ and its verb spielen ‘to play’ and its agent noun der Spieler ‘the player.’ English has borrowed words like glockenspiel ‘bell-play,’ an instrument in which little bells suspended from a frame are struck by hammers. From German through Yiddish, spiel meaning a rapid-fire sales pitch came into English.
All are relatives of the English word spell ‘a magical charm spoken to put someone under the power of another,’ hence the original meaning of spellbound, an adjective where the roots of bonspiel are reversed, so to speak. A spell was a very special something spoken. In Middle English spell meant speech, talk, tale, as it did in Old English. Spell is related to Old High German spel ‘tale’ and to Old Norse spjall ‘story’ and to Gothic spill ‘talk.’
Yet another related word is spelling. When you spell a word, you ‘talk’ out its letter aloud. That verb to spell comes from Middle English spellen, from Old French espeller, which is of Germanic origin and is related to Old English spellian ‘to relate’, ‘to talk,’ and Middle High German spellen and Old Norse spialla ‘to talk’ and Gothic spillon ‘to relate.’
Perhaps the most surprising relative to show up on the doorstep of bonspiel is the word gospel ! But it’s true. In Old English godspel was ‘good talk,’ the good tale, the good news about Jesus in the first four books of the New Testament.
In linguistics, gospel is referred to as a loan-translation, that is, early Latin-speaking monks took an ecclesiastical Latin phrase like bona annuntiatio or bonus nuntius ‘good message’ and translated the meaning into Old English, using English roots instead of Latin ones, but keeping the same order of root words as in Latin.
After all that, I need a spell of rest.
Sorry, the restful spell is a word from a root different than those terms discussed above.
Gott sei dank!

Winnipeg, Manitoba, Canada
Bonspiel Poster