plying their trade

病院の食事は軽く、手術の日は丸一日絶食、もっとも点滴で栄養は摂っていますが、それで体重が減ったかと思いきや、反対に1.5も増えていました。手術前を含め3週間程卓球とテニスを休んでいるのが原因の様です。退院時に後2週間はスポーツはできないと医者に言われているのでまだまだ体重が増えそうです。しかし丁度オリンピックの最中なので退屈はしません。
さて、入院前ですが、サンデル教授のThe Case against Perfectionを読みました。
In India, the number of girls per 1,000 boys has dropped from 962 to 927 in the past two decades. India has banned the use of prenatal diagnosis for sex selection, but the law is rarely enforced. Itinerant radiologists with portable ultra-sound machines travel from village to village, plying their trade. One Bombay clinic reported that, of 8,000 abortions it performed, all but one were for purposes of sex selection.
"plying their trade"の "ply" は昨年取り上げていましたが、今回の意味あるいは用法ははちょっと違います。
Collins English Dictionaryを見ると、
1.to carry on, pursue, or work at (a job, trade, etc)　
3.to sell (goods, wares, etc), esp at a regular place
6.also intr (esp of a ship) to travel regularly along (a route) or in (an area) ⇒ to ply between Dover and Calais, to ply the trade routes
とあり、前回は6の例に近かったのですが、今回の意味は上の1と3に近いと思います。いつものように他の辞書も見ます。
・Vocabulary.com:apply oneself diligently: “Ply one's trade”
Ply means to work steadily at something. If you've got a lemonade stand and you're busy perfecting your recipe and seeking out customers, you're doing a good job of plying your trade.
A ship that regularly travels a particular route plies that route. When someone offers you food or drink, often in hopes of getting something in return, they're plying you: "If I ply you with cake, will you take my dog for a walk?" The noun ply is very different from the verb, referring to layers or strands. Four-ply yarn is made up of four strands. Plywood is made of many layers of wood glued together.
流通が発達したので今はほとんどいませんが、"ply"の意味はほぼ絶滅した行商を思い出すと良いかも知れません。