Tā duì zhè jiàn shì biǎoxiàn de dàndàn de, sìhū bù tài zàiyì.
He seemed indifferent about the matter, as though he didn't care much.
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usage
淡淡 is the reduplicated form of 淡 (light/bland), adding a gentle, softened nuance. It can describe sensory impressions (light smell, dim light) or emotional states (mild indifference).