to prepare your muscles for a physical activity by stretching and doing simple exercises

I do a few stretches to loosen up before I run. 我跑步前伸展四肢热了热身。

to start to feel less embarrassed and to become more relaxed when you are with other people, or to make someone feel like this

He seemed quite nervous at the beginning, but he soon loosened up. 刚开始的时候他似乎非常紧张，但不久就放松了下来。

A gin and tonic will loosen you up. 喝一杯加了奎宁水的杜松子酒能缓解你的紧张情绪。