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

 

tighten up

 phrasal verb
phrasal verb
 
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Phrasal Verbs

tighten up (on something)

 
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to become stricter or more careful Laws on gambling have tightened up recently. The police are tightening up on under-age drinking.
See the Oxford Advanced American Dictionary entry: tighten up