The noun ρεδα (reda) describes a four-wheeled carriage or chariot. This word is remarkable as it is wholly foreign to Greek (there aren't even any Greek words that start with ρεδ-, red-), and appears in the whole of extant Greek writing in Revelation 18:13 only. The Byzantine Majority text spells this word as ραιδα (raida), or rather its genitive neutral plural form ραιδων (raidon), which is also the form in which A Greek-English Lexicon of Liddel and Scott lists it.

This particular form ραιδα (raida) may perhaps have reminded people of the word ραιδιον (raidion), a diminutive of the adjective ραδιος (rhadios), easy or easygoing, from the adverb ρεα (rea), easily or lightly. Conversely, there is the verb ραιω (raio), to smash or destroy; hence the noun ραιστηρ (raister), smasher, the name of the smith's heavy hammer.

But otherwise, it's a mystery why the Revelator would choose to deploy a word from another language, as Greek had a wide range of words for a wide range of carts and chariots (the noun αρμα, arma, comes to mind). And even if pressed, John's audience would probably have been more familiar with the common Hebrew word for chariot: מרכבה (markaba). Instead, John used ρεδα (reda), which stands out in the Greek text the way the term Lastkraftwagen (that's German for truck) would have stood out in anything written by the Allies in 1944. Ergo, John specifically points toward the Romans and their commercial cargo vehicles.

Our noun ρεδα (reda) is a transliteration of a word that was relatively common in Latin, namely reada, which described a heavier four-wheeled cart, which could haul up to 340 kilograms of cargo (or passengers) and was pulled by eight horses in the summer and ten in the winter. The Roman Empire was an empire of roads (the granddaddy of which was the Appian Way) and vast convoys of readae thundered over them, at all times and in any direction. It's perhaps not too much of a stretch to suppose that the reada had become a symbol of the perpetual economy of the city that never slept, and whose very existence was a permanent violation of the Lord's Sabbath.

Our word is also not native to Latin, but, especially in its variant spelling of reda, perhaps somewhat resembled the very common verb redeo, to return. It's ultimate source appears to be Celtic, which received it from the same Proto-Indo-European root "reyd-", to ride, from which English inherited words like ride, road and raid.