He maintained a buttoned-down attitude during the meeting, rarely showing his true thoughts.

buttoned-down 是由动词短语 button down（扣上纽扣）演变而来的形容词。button 指"纽扣"，down 表示"向下"，字面意思是"扣紧纽扣"。 这个短语引申为"拘谨的、克制的"，形容人言行谨慎、不轻易表露情感的状态，就像扣紧的纽扣一样严丝合缝。