GOP stands for “Grand Old Party,” and it has been associated with the Republican Party since the 1870s, which is a little ironic given that the party was only formed in 1854 and by any standards was not yet old. In fact, the Democrats used the phrase “Grand Old Party” in 1859 and 1860 to describe their party but dropped the moniker, perhaps because the 1860 presidential election didn’t work out particularly well for them.
None of that, however, explains how “Grand Old Party” got shortened to GOP. The answer lies in the vagaries of typesetting. In 1884 the Republicans had nominated James G. Blaine as their presidential candidate, and T.B. Dowden was a young typesetter for the Cincinnati Gazette. He was on deadline for the newspaper’s press run and had one line of type more than would fit on the page. He abbreviated “Grand Old Party” to GOP to make everything fit. While Grover Cleveland went on to defeat Blaine and relegate him to the annals of history, the shorthand name of the Republican Party—the GOP—became a political fixture.