Jī jiǔ de yuànduì zhōngyú huà wéi gōngkāi de díyì.
Long-stored resentment finally turned to open hostility.
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In this classical reading 怼 means 'resentment / to hate', literary and mostly in 怨怼. A separate spoken reading 怼 is the popular internet word for 'to clap back at someone'.
Top 对 gives the reading duì; its 'facing / opposed' sense also colors the meaning — resentment is set against someone. Traditional 懟 uses the fuller 對.