The word slogan was first attested sometime in the 1670s, but it was around since the 1510s in the form of slogorne. These terms come from Gaelic sluagh-ghairm, which meant "battle cry". This is rather surprising, but makes sense if you consider the context: slogans used picked up by political factions, which ranged from violent to peaceful. By 1704, the word adopted a secondary definition of "distinctive phrase", something that was later applied to companies as well. Sluagh is from the Old Irish word slog, also "army", and that (through Proto-Celtic slougos) eventually is from Proto-Indo-European slowgo, or "entourage". Ghairm, meanwhile, hails from Old Irish gairm, "cry", which (through Proto-Celtic garman) eventually derives from the Proto-Indo-European reconstruction ghr, also meaning something like "shout". I just think that's an awesome and unexpected etymology!
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.