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on (one's) back
Constantly nagging one about something. Ugh, my mom is on my back all the time about my grades, but I just have no interest in the classes I'm taking this semester.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
on (someone's) back
Persistently nagging or urging someone to do something.
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.