Part of the point (the whole point?) of rhyming slang is that it’s opaque and convoluted. That’s what makes it fun. It also makes it a bit of a shibboleth - you only understand it if you’re part of the culture.

If you’re eating with an Australian and they ask you to “pass the dead horse”, it means they think you’re Aussie enough to know what it means. Or they know that you won’t know what it means and they’re fucking with you intentionally.