A person who rides your back so hard you need to pry them off. Often a co-worker, teacher or manager in professional settings but could also be a significant other, semi-friend or admirer in social settings.
Rick was tagging along with us at the club last night. He just followed along behind me all night. I'm not taking him out anymore. He's such a backpack!
My boss just looms over my desk all day. He's constantly riding me. What a backpack!