What It Means:
"after this, therefore on account of it"
Where It Comes From:
Post hoc, ergo propter hoc refers to the logical mistake of claiming that one thing caused another just because it happened first.
For example, the rooster crows and the sun rises but to argue that the rooster's crowing causes the sun to rise would be post hoc, ergo propter hoc reasoning. A more familiar example might be when politicians take credit for improvements that occurred after they took office as if their policies necessarily caused those improvements.
(It was Aristotle who laid the groundwork for classifying bad arguments based on logical errors like this one.)