The word guitar was first used in a 1637 translation of Horace's Art of Poetry. Obviously, there were no modern-day guitars in classical Rome; at the time, the word probably referred to a lute, or was just a fanciful translation. Guitar comes from Spanish guitarra, which reflects the instrument's origin in fifteenth-century Iberia. Before that, it might come from Arabic qitara, but it definitely (either directly or indirectly) derives from Latin cithara, which described a lyre-like instrument used primarily in Ancient Greece (this also gave us the word zither). In the original Greek, that was kithara, and it's probably Proto-Indo-European because of a cognate with sihtar, the Persian etymon of the word sitar. The portmanteau keytar is from 1979 and the rare verb to guitar was first used in 1816.
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.