Other forms: plying, plied, plies, plys
To ply a route is to travel it regularly — ferries plying between the islands, taxis plying for hire. To ply a trade is to work at it steadily.
Both are formal and slightly old-fashioned, and both turn up in writing about transport and traditional crafts. There is a third use worth having: to ply somebody with something is to keep offering it insistently — ply them with questions, ply them with drink. As a noun, ply means a layer or thickness: three-ply wool, plywood, two-ply tissue, which is where most people meet the word first.
1verbplyTo travel a route regularly.
“Two ferries ply between the mainland and the island.”
2verbworkTo work steadily at a trade.
“He plied his trade in the same workshop for forty years.”
3nounlayerA layer or thickness of something.
“Use three-ply wool for the sleeves.”
4verbkeepTo keep offering somebody something insistently.
“They plied him with questions until he left.”
ply is a way of working, or a layer.
The Latin is to fold. One ply is one fold of the cloth, and a boat that plies a route folds back and forth along it.
hooks ontofold
The layer sense comes from Latin plicare, to fold — the same verb behind pleat, apply and complicated. The travelling sense is a shortening of apply, in the old sense of applying yourself to a route or a trade, so the two meanings are the same word by two different routes.
Proto-Indo-European
*pleḱ- — to fold, plait, weave
Middle French
pli
Anglo-Norman
pli
Middle English
pleit
English, today
ply
Built from
From Middle English pleit, plit, plite (“a fold, pleat, wrinkle; braid, strand in a braided cord, ply”), from Anglo-Norman pli, plei, pleit, and Middle French pli, ploy, ply (“a fold, pleat; joint in armour; situation, state”) (modern French pli (“a fold, pleat”)), from plier, ployer (“to bend, fold”), from Latin plicō (“to bend, fold, roll up”), from Proto-Indo-European pleḱ- (“to fold, plait, weave”).
Etymology from Wiktionary, CC BY-SA