1. slang To inform against a criminal or wrongdoer to a figure of authority. A noun or pronoun can be used between "grass" and "up." Primarily heard in UK. It turned out the drug dealer was under police protection, as he had been grassing up his suppliers for years. Why would you grass me up to the teacher like that? Why would you grass that guy up? You know he'll just send his thugs around to beat on you!

2. To add elements of bluegrass music to a song, or to perform a song in the style of bluegrass. A noun or pronoun can be used between "grass" and "up." Primarily heard in US. We brought in a few banjo players to grass up the song a bit. The guitarist recorded a new version of the famous song, grassing it up a bit to make it sound her own. Whew, I can't play my guitar fast enough to really grass up that song!