He politely declined the dinner invitation, saying he was too busy.

declined 是动词 decline 的过去式。decline 由前缀 de-（表示"向下"）和词根 cline（倾斜）组成，本义是"向下倾斜"，引申为"婉拒、谢绝"的意思。在这里指礼貌地拒绝邀请。