To harm, damage, or act contrary to the interests of something; to do a disservice to.
From Old French desservir (des- 'un-' + servir 'to serve'), meaning 'to serve badly' or 'to act against.' The prefix dis- here means 'opposite' or 'negative.'
Interestingly, 'disserve' literally means 'to un-serve'—the opposite of the loyal, helpful 'service'—which is why saying 'you did him a disservice' implies betrayal of an expected loyalty.
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