Definition of pick up on phrasal verb from the Oxford Advanced Learner's Dictionary

 

pick up on

 phrasal verb
phrasal verb
 
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Phrasal Verbs
  1. 1to notice something and perhaps react to it She failed to pick up on the humour in his remark.
  2. 2to return to a point that has already been mentioned or discussed If I could just pick up on a question you raised earlier.

pick somebody up on something

 
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to mention something that somebody has said or done that you think is wrong I knew he would pick me up on that slip sooner or later.
See the Oxford Advanced American Dictionary entry: pick up on