Gone to grass. Dead. The allusion is to the grass which grows over the dead. Also, “Gone to rusticate,” the allusion being to a horse which is sent to grass when unfit for work.
“Captain Cuttle held on at a great pace, and allowed no grass to grow
under his feet.” —
To give grass. To confess yourself vanquished.
To be knocked down in a pugilistic encounter is to “go to grass;” to have the sack is also to go to grass, as a cow which is no longer fit for milking is sent to pasture.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894