BATTLE CRY

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!