v.1. To make something tight or tighter: I pulled on the ends of the string to tighten up the knot. The mechanic tightened the bolts up with a rachet. This exercise will tighten up your stomach muscles. Tighten your belt up so your pants don't fall down.
2. To become tight or tighter: I knew I had a fish when the line suddenly tightened up. After the run, I walked around the track so my muscles wouldn't tighten up. I tightened up on the handlebars as I went over the bump.
3. To make something more strict or secure: The government is trying to tighten up the tax code. The country has tightened its borders up to prevent drug smuggling.
4. To become more strict or secure: Airline security has tightened up, and now all luggage must be scanned.
5. To make something more disciplined: The company is tightening up their management in an effort to reduce wasteful spending. The author has tightened the story up by deleting irrelevant details.
6. To become more disciplined: The team has tightened up under the leadership of the new coach.