put on side
dated To think or behave as though one is superior to others or better than one really is; to have or assume a pretentious or self-aggrandizing attitude. Primarily heard in UK. I think John is really a decent fellow at heart, but I really wish he wouldn't put on such side about his writing abilities. Stop putting on side, Mary. You're not some world-class actor—you're just an amateur like the rest of us!
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