The accepted figurative meaning of this is that men cannot change their
characters, and just as a leopard is born spotted and will always be so, a man
with a wicked nature is born wicked and will never be anything else. The more
exact meaning is that, although not basically wicked, a man can become in the
course of time so degraded and sung in ungodliness that there is no more
changes of changing his ways than there is no more chance of changing his was
than there is of a leopard’s changing his spots.

The source is Jeremiah, xiii, 23; ‘Can the Ethiopian change his skin, or the
leopard his spots?’