1Scottish, British dialect Worthless material; trash, rubbish, refuse. Also occasionally: miscellaneous material, odds and ends.
2People of the lowest social class considered collectively; disreputable or undesirable people.
3With the: the most disreputable or the lowest element of a class, community, etc.; the common run of people; the rabble.
4A base, worthless person; a member of the riff-raff.
Late Middle English (in an earlier sense). Apparently partly short for riff and raff one and all, everyone, everything, and partly (especially in later use) shortened from riff-raff.