"You look like you've just crawled out of a poppy field" —Glinda to West
Poppy aka Opium is a drug in Oz that can get a person high. The poppy pollen is what makes the Brick Road yellow.
The Cardinal Witch West is addicted to poppy tea. She drinks it to take the edge off whenever she can, most notably before performing a funeral ritual for her sister, East. The process of preparing the poppy tea makes one's fingers skin darken, so dark fingertips can be a sign of an addicted person.
In real life, opium is a dried latex from opium poppy. It's scientific name is Lachryma papaveris, poppy tears.
The poppy is based on the poison poppy fields from the original 1900 novel.