For sale; being offered or available for purchase; in a state of being sold.
From a- (preposition prefix) + sale (from Old Norse sala 'to sell'). Old construction similar to 'aflame' or 'ablaze' indicating a state or condition.
Words like 'asale' show how English once used a- as a flexible prefix to indicate states—we've mostly abandoned this, which is why 'on sale' sounds more natural to modern speakers than 'asale'.
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