noun
A shirt worn with the top buttons undone, leaving the neck open. Wasei-eigo from English 'open shirt'.
wasei-eigo (Japanese-coined English), from open shirt
夏はオープンシャツで通勤する人も多い。
In summer, many people commute in open-necked shirts.
彼は白いオープンシャツを着て、
He was walking coolly in a white open-necked shirt.