to be able to eat or drink something without vomiting

On the day after her operation she couldn't keep anything down. 手术的第二天，她吃什么都吐。

to control the size or number of something and prevent it from increasing

We need to work hard to keep our prices down. 我们必须努力控制我们产品的价格。

I'm going to sleep, so will you try to keep the noise down? 我要睡觉了，你能不能不要太吵？

to prevent a person or group of people from having any power or freedom

It's all part of a conspiracy to keep women down. 那完全是压制女性自由的部分阴谋。