Picture a road that is open at both ends. Something can travel toward you and away from you along the same line, at the same time. Once you hold that picture, the other uses fall into place: the thing travelling can be cars, it can be goods, it can be words on a radio, it can be effort between two people, or it can be a player working at both ends of a court.
可以想成一條兩頭都通的路。東西可以朝你過來，也可以離你而去，走的是同一條線。抓住這個畫面，其他用法就會自己就位：在路上跑的可能是車，可能是貨，可能是無線電裡的聲音，可能是兩個人之間的付出，也可能是球場上那個兩端都要顧的球員。每次看到 two-way，先問一句：是哪兩個方向。
From the word’s history
Wiktionary derives it from two + way, and compares Old English twiweġ, 'an intersection, a junction of two roads'. The road picture is therefore already in the parts of the word.
維基詞典說 two-way 來自 two 加 way，並拿古英文的 twiweġ 作比較，那個字指「兩條路交會的地方」。也就是說，「路」的畫面本來就藏在這個字的組成裡，今天的各種用法都還看得到它。
Wiktionary 'two-way'（封包 source_ref kaikki:3f6f69a190036ae4:1148693094）
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