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.
noun word-forming element meaning "one who" (operates, produces, deals in); Englished form of French -ier, from Latin -arius, -iarius; compare -ary. Usually in English words of more recent borrowing from French; older words tend to keep -ier.
<a href="https://www.etymonline.com/word/sloganeer">Etymology of sloganeer by etymonline</a>
Harper, D. (n.d.). Etymology of sloganeer. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/sloganeer