That man took off his shirt and danced in the park, what an exhibitionist.

exhibitionist 是由名词 exhibition（展示、展览）加上后缀 -ist（表示"从事某种行为的人"）构成的名词，指"喜欢在公共场合展示自己身体或行为以吸引注意的人"，即"暴露狂"或"爱出风头的人"。