Think of a line drawn round the lit stage. Inside it, the audience is looking; that is where the play happens. Offstage is the strip just outside that line, at the sides and behind: still inside the theatre, often only a step away, but out of everyone's view. Everything this word does comes from standing on the far side of that line.
可以想成舞台邊上有一條看不見的線。線裡面是燈光打亮的地方，觀眾的眼睛只到這裡。往兩側或往後跨一步，人還在劇場裡，常常只離舞台一兩公尺，卻已經不在任何人的視線裡。offstage 指的就是線外那一邊：東西還在，只是沒被看見。這個字後來的每個用法，都是站在那條線的外側往回看。
From the word’s history
The packet records the word simply as off + stage, which is the picture itself: away from the stage.
資料來源把這個字記成 off 加 stage，字面就是「離開舞台」，正好就是它的核心畫面。
Wiktionary, "offstage"
登入就可以看每一支怎麼連回核心畫面。