1670s, earlier slogorne (1510s), "battle cry," from Gaelic sluagh-ghairm "battle cry used by Scottish Highland or Irish clans," from sluagh "army, host, slew," from Celtic and Balto-Slavic *slough- "help, service." Second element is gairm "a cry" (see garrulous). Metaphoric sense of "distinctive word or phrase used by a political or other group" is first attested 1704.