Definition of run in phrasal verb from the Oxford Advanced Learner's Dictionary

 

run in

 phrasal verb
phrasal verb
 
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Phrasal Verbs
(old-fashioned, informal) to arrest somebody and take them to a police station
(British English) (in the past) to prepare the engine of a new car for normal use by driving slowly and carefully (figurative) Whatever system you choose, it must be run in properly.
See the Oxford Advanced American Dictionary entry: run in