Well the simple past tense / 3rd person of "chier" is precisely "chia", spelled exactly like that. The herb/plant "chia" always sounds a bit weird to my ears because it makes me think of the verb tense... "Il chia" / "He shat". So GP is totally correct.

GP is maybe “technically” correct, but given that «chier» is a slang verb mostly used orally while passé simple is almost never used orally and never in casual context, which means that you'll never use that tense on this verb in practice. “He shat” will 99.99% of the time be translated to «il a chié» (passé composé).

Thanks. I’ve never in my life heard “il chia”. And you wouldn’t formally write something so informal.

Anyone curious can verify that google translate doesn’t even understand “il chia” and google translate is quite good on French.

It's precisely the juxtaposition of high grammar and low slang that makes it so humorous.

Someone cares enough to have compiled the conjugation: https://la-conjugaison.nouvelobs.com/du/verbe/chier.php

and it's a really old word, used frequently by Rabelais, for instance.