“Shroom” is an informal shortening of “mushroom”. Merriam-Webster labels it informal and gives two senses: any mushroom, and especially a fungus, such as one of the genus Psilocybe, that contains hallucinogenic alkaloids such as psilocybin (Merriam-Webster). It also lists the apostrophe spelling ’shroom as a less common variant.
The dictionaries disagree on its age. Merriam-Webster dates the first known use to 1976 (Merriam-Webster), while the Collins entry shown on Dictionary.com marks its origin as the 21st century (Dictionary.com). Both agree it comes from “mushroom”. Merriam-Webster gives the pronunciation as rhyming with “room” with “sh” in front.
Context decides the meaning. In a recipe, shrooms can just be the mushrooms on a pizza; in talk about drugs, the word usually means mushrooms that contain psilocybin. Science and law do not use the slang: they name the compounds, psilocybin and psilocin, or the species. Calling something shrooms says nothing about which species it is or what it contains, and many unrelated fungi share the nickname.
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