To deprive someone of their gallant qualities or behavior; to make someone less courtly or chivalrous.
From dis- (meaning 'reverse' or 'deprive') combined with gallant (from Old French gallant, meaning 'spirited' or 'courtly'). This word reflects medieval and Renaissance concerns with courtly behavior and chivalry.
This word is almost extinct because the whole concept of 'gallantry' as a measurable quality requiring formal behavior faded away—you can't disgallant someone if gallantry itself isn't a culturally enforced standard anymore.
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