Past tense of affy; to betroth or pledge oneself in marriage; to become engaged or betrothed.
From Middle English affien, from Old French affier, derived from Latin ad- 'to' + fidus 'faithful.' The sense evolved from 'to pledge faith' to specifically mean pledging oneself in marriage or becoming betrothed.
In medieval literature like Chaucer, characters get 'affied' to each other—it meant they were officially engaged, which was almost as binding as marriage itself, and breaking an affiance could result in legal punishment or having to pay damages.
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