Slippery Slope
Thin End of the Wedge · Domino Fallacy
Asserting that if we allow A to happen, then Z will eventually happen too, therefore A should not happen.
Origin
British logician Alfred Sidgwick was the first to describe this argument pattern in his 1910 book The Application of Logic, noting that people often argue "we must not do this, because if we did we should be logically bound to do something else which is plainly absurd." The specific English phrase "slippery slope" in its argumentative sense was first recorded around 1951, though the underlying reasoning pattern traces to classical rhetoric and Aristotle's catalogue of fallacies in Sophistical Refutations.
Everyday Use
"If we let employees work from home on Fridays, soon they'll want the whole week off." You hear this reasoning everywhere — in politics, parenting, and office debates. It assumes that one small step will inevitably lead to an extreme outcome, skipping over all the decision points in between where people could simply say no.