Every advertising slogan is, by origin, a Highland war cry.
The source is Scottish Gaelic sluagh-ghairm. Merriam-Webster and Etymonline both split it into sluagh, "army" or "host," and gairm, "cry." Etymonline describes it as the battle cry of Scottish Highland or Irish clans.
The Scottish National Dictionary is more specific. A slogan was usually the name of a clan chief or of the clan's meeting place. It was shouted to gather armed men, or used as a password. For the Buchanans, a 1723 account says, the cry was the name of the island of Clareinch.
The word entered Scots writing early. Merriam-Webster and the Scottish National Dictionary date it to 1513, and Etymonline credits the spelling slogorne to the poet Gavin Douglas in the 1510s. Later spellings include slogane (1571), slogurn and slughorn. Sir Walter Scott, the dictionary notes, did much to popularize it. "The Border slogan rent the sky!" he wrote in Marmion in 1808.
One of those spellings produced a famous mistake. Thomas Chatterton misunderstood "slughorn" as a kind of horn and used it that way in his poem "Battle of Hastings." Robert Browning, who admired Chatterton, inherited the error. "Childe Roland to the Dark Tower Came" ends with the hero setting "the slug-horn to my lips" and blowing. No such instrument exists. The critic John Kerrigan called it "a howler," though one with "a peculiar rightness" in the poem.
Meanwhile the word was moving from battlefield to politics. By 1704 it meant a phrase that identified a political or other group, in a line attacking opponents: "Your slughons are falsehood and plunder." The Scottish National Dictionary says the word was borrowed into English in the mid-19th century, apparently in this motto sense.
The last shift was commercial. Merriam-Webster now gives a separate sense for a short, eye-catching phrase used in advertising. English also built a family around it: sloganize (1909), the noun sloganeer (1922) and the verb sloganeer (1941). The clan cry that once called men to fight now calls customers to buy.