His acting was so stagey that it seemed deliberately theatrical.

stagey 是形容词，由名词 stage（舞台）加上后缀 -y 构成。-y 后缀常表示"具有...特性的"或"像...一样的"。stagey 形容表演或行为"像舞台表演一样夸张、不自然"，带有贬义，指"做作的、刻意戏剧化的"。