Apparently thinking about "the shrews" or "rooster's balls" or "the maternity of giraffes" or "pregnant birds" are also used.

Somebody claims it's from Greek mythology where a crab is talking to Zeus and claiming he's immortal because he doesn't just walk forward but in any direction (time-travel like). I couldn't find that myth, but there is one in which he gets... killed by Heracles for pinching his foot in a battle with the Hydra and gets placed among the stars by Zeus's wife as a reward for loyalty.

If there is an origin to the saying I don't think anyone knows it or thinks of it anymore. It's just a ridiculous/inexistent thing to think about, like those other examples above.