hubieran calumniado
-they/you had libelled
Past perfect subjunctive ellos/ellas/ustedes conjugation of calumniar.

calumniar

Confiábamos en que, durante ese lapso, los Vitaver nos hubieran calumniado lo suficiente para disuadir a cualquiera de invitarnos a otra fiesta.
During that interval, we have relied on the Vitavers to slander us enough to dissuade anyone from inviting us to another party.
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Christmas carol