"To grass" on someone or some group is to be an informer. So if a grass is an informer, to grass, grassing or grassing up someone describes the act of informing. When you grass on someone or something, you are not only filling the role of informer but also of the betrayer.

What is grass in slang?

To grass in British slang is indeed to inform on a person to the authorities; a grass is an informer. ... It has been proposed that grass is from snake in the grass, a treacherous person or a secret enemy.

What does it mean to feel grass?

People on the internet can be found saying this as an insult to people who they think are very disconnected from real life: "You seriously need to go outside and touch some grass."