Archaic second person singular form of 'chance,' meaning you might or you happen to do something.
From Old English 'chansan,' from Old French 'cheance.' The '-st' ending was a standard archaic English marker for second person singular verbs.
Words like 'chanst' show how English used to conjugate verbs completely differently—we've lost all these '-est' and '-st' endings, which is why 'thou chanst' sounds so strange to modern ears.
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