An alternative or archaic spelling of 'gain,' meaning profit, advantage, or something acquired through effort or fortune.
From Old Norse 'gagn' meaning benefit or profit, entering Middle English through Viking contact and trade. The 'e' ending is an archaic spelling convention no longer used in modern English.
You can actually trace English's Scandinavian DNA through words like 'gaine'—Vikings not only invaded England, they basically rewired the language, giving us money-and-profit vocabulary we still use today.
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