Answer:
The word cargo is from the 1650s, "freight loaded on a ship," from Spanish cargo "burden," from cargar "to load, impose taxes," from Late Latin carricare "to load a wagon or cart," from Latin carrus "wagon" Cargo is also used in air and rail travel.
The words have origins in different languages, so that English has applied meanings to them which don't necessarily correlate with their memes.
"Cargo" derives from a Spanish word meaning "load" which was derived from the Latin "carrus" a wheeled vehicle, which survives in English as "cart."
"Shipment" derives from the Old English word "scip" pronounced the same as modern "ship" and having cognate forms in other Germanic languages. The historical usage of "shipment" was to indicate the totality of cargo being transported at one time, whether on multiple wagons or multiple ships.
As others have pointed out, recent usage--since hardly anyone thinks about the wagons or ships involved--is to consider them close synonyms and ignore their separate histories.
Explanation:
