In the previous post, we learned hundred literally means “count of 100.” How about the next multiple of ten up the scale, thousand?
The word thousand comes from the Old English þúsend, which the Oxford English Dictionary attests in the record as a early 971. (That þ symbol is a letter called “thorn,” and English once used it for th sounds.) The Old English þúsend has cognates across Germanic languages (Dutch duizend) and, as the evidence suggests, in many Balto-Slavic ones, too (Lithuanian tūkstantis).
Indo-European etymologists think thousand and its numerous kin literally means “swollen hundred.” The first part, they propose, comes from the Proto-Indo-European *teu-, “to swell,” which also yields the English thigh. The second part, as they see it, is related to the same root behind the hund- in hundred (*dkm-tom).
But what was a “swollen hundred” for the ancient speaker? It probably had the sense of “a great multitude.” Noting that there is no common Indo-European word for thousand, historical linguists suspect the Latin and Greek for “thousand”—mīlle (millennium, million) and khīlioi (kilo), respectively—both began as indefinite terms for very large numbers. The word myriad behaves similarly in modern English, from a Greek root for “ten(s) of thousands” or simply “countless.”