The word armadillo was borrowed in 1577 from Spanish, where it was a diminutive of armado, the word for "armored". This implied that armadillos are small armored animals - very appropriate. Armado comes from Latin armatus, which meant "armed" and is the past participle of arma, or "weapons". More literally, it translates to "tools of war", because in its earliest days, arma had a definition of "tools". Finally, everything is reconstructed to Proto-Indo-European ar, meaning "fitting", on the notion that some tools fit things together. Arma also gave us words like armory, armament, Armageddon, armada, army, alarm, and armoire, so there are some interesting cognates there. According to Google NGrams, usage of the word armadillo has remained relatively constant since it was first popularized in the late eighteenth century.
Adam Aleksic, a rising sophomore studying linguistics and government at Harvard University, has been described as the internet's sixth most famous etymologist. He also has disturbing interests in politics, vexillology, geography, board games, conlanging, and law.