the.com/slippery slope

means A reasoning pattern — often fallacious — that claims one small step will trigger an unstoppable chain of events leading to a drastic, usually bad, outcome, without showing why each link in the chain must follow.

from Built from the plain, literal image of a slope too slick to stand on — one footing lost and you slide all the way to the bottom, unable to stop partway. That physical metaphor got borrowed into argument and ethics writing, where it's used to describe reasoning that treats a first step as if it mechanically forces every step after it. The phrase as a named fallacy/rhetorical label is generally traced to 20th-century usage in legal, philosophical, and moral debate, though the 'slippery' metaphor for moral decline is older and more diffuse — so treat the exact coinage date as fuzzy rather than fixed.