That expression comes from the days when people slept on beds filled with dried grass or hay.

VOA: special.2009.09.20

They picnic and they barbecue, and just sit out on the grass and just relax. You know it's a beautiful park.

他们在那里野餐，露天烧烤，或者简单地坐在草地上放松。那是一个很美的公园。

河滨公园 - SpeakingMax英语口语达人

It's for 3 or 4 days and it's put on by a local businessman who's a blue grass freak,

它会持续3到4天，是当地的一名商人赞助的，他是个热爱蓝草的怪人，

愉快的音乐庆典 - SpeakingMax英语口语达人

"We've taken our lead from the women on the ground. This is a grass roots program.

VOA: special.2010.06.16

so you play on a grass as opposed to on the clay

所以，你在绿茵场上比赛，而不是在粘泥土上，

世界级网球比赛 - SpeakingMax英语口语达人

Her feet made no sound on the grass, but he turned suddenly to face her.

VOA: special.2009.12.12

A piece of sod has grass on the top and soil on the bottom.

VOA: special.2009.07.07

On top of the wood, he put some grass and dry branches.

VOA: special.2010.05.01

It is the color of grass and the leaves on trees.

VOA: special.2010.05.23

Opponents have already spent millions of dollars on television ads aimed at whipping up grass-roots opposition to the Obama plan.

VOA: standard.2009.07.31

"For example, green turtles feed on sea grass, so they bring the balance between the sea grass ecosystem and also the other species that are existing in the sea grass ecosystem.

VOA: standard.2010.07.15

"We are at 100 percent of our planning level in terms of protection of Gulf islands,national seashore, to protect predominantly the wetlands and estuaries and sea grass beds that are on the backside of the islands."

VOA: standard.2010.05.26