Sleepy grass is a type of secret coating used to knock people out. It is a small substance that is placed in food or liquid to be consumed by the victim. Sleepy grass is used twice in the anime, in the Emperor Pilaf Saga and in the Dragon Ball Z episode "Princess Snake".
In the Emperor Pilaf Saga, Oolong tried to knock Goku out so he could take his and Bulma's Dragon Balls for himself, but was foiled by the interference of Puar and Yamcha. Princess Snake tried to use this on Goku, but it only worked for a few minutes.