type of heavy, single-edged sword, usually slightly curved, 1670s, from French sabre “heavy, curved sword” (17c.), alteration of sable (1630s), from German Sabel, Säbel, which probably is ultimately from Hungarian szablya “saber,” literally “tool to cut with,” from szabni “to cut.” The Balto-Slavic words (Russian sablya, Polish szabla “sword, saber,” Lithuanian šoblė) perhaps also are via German, but Italian sciabla seems to be directly from Hungarian. Saber-rattling “militarism” is attested from 1922. Saber-toothed cat (originally tiger) is attested from 1849, so named for the long upper canine teeth.
EDIT: Oooh, etymonline is wrong (or at least not complete). Mirriam-Webster has earlier known uses, says that it was used in the UK first, around the late 1870s.
There is no unanimity of opinion on why we came to refer to this kind of behavior as saber-rattling. Some think that it comes from the practice of 18th-century Hungarian cavalry units had of brandishing their sabers at opponents prior to charging. Others have said that it comes from the habit that military officers had in the early 20th century of ominously shaking their scabbard when issuing orders to subordinates. Our records indicate that the two words began seeing use in fixed fashion around 1880, making it unlikely that it was directly related to either of the causes given above.
Of late it has been in some quarters impossible to mention the word patriotism without having the taunt of being a sabre-rattling BOBADIL thrown in one’s face.
— The Standard (London, Eng.), 19 Feb. 1879
The “Sabre Rattling” of M. Coumoundouros, especially his assertion that by the coming spring he will have 86,000 men in the field, and that this number of troops will have been got together by the 10th of December.
— The Leeds Mercury (Leeds, Eng.), 3 Nov. 1880
The word appears to have begun in the press in the United Kingdom first, and by the early 20th century had spread to newspapers in the United States.