I looked it up when I saw it, it seems like it comes from a fable with a similar moral to “If You Give a Mouse a Cookie”

A camel sticks his nose under the edge of a tent for warmth, it’s owner (or a stranger w/e) allows it to out of kindness. Then the camel slowly worms it’s entire body in the tent and refuses to budge.

It’s a slippery slope parable.

In a sane society Trump should be in prison.

I’ve never heard that phrase before that’s an excellent phrase.