This phrase often implies annoyance and a lack of patience from the person complaining or nagging. Stop being on my back about cleaning my room! I'll do it later.
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Colloquial Expression
Primarily used in casual speech among friends or family, not in formal situations. She's always on my back to quit smoking, but I appreciate her concern.
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Emotional Weight
The phrase can express the speaker's feelings of being controlled or overwhelmed. My brother is always on my back about borrowing his things without asking.