The Malay phrase 'terima kasih daun keladi' literally means 'thank you yam leaf', and it seems to be used very regularly. A Google search for the phrase gives 1,340,000 results. Now, I haven't checked them all, but certainly the first few involve the phrase exactly like that.
Now, why do Malays say that? It doesn't seem to make any sense. My colleague, Malai Ayla, tells me it is the first line of a pantun, a short rhyming ditty in Malay, and my colleague Adrian Clynes confirms that pantuns don't always make much sense. They are just bit of rhyming fun in Malay.
Further investigation suggests that the next line of the pantun is 'kalau boleh hendak lagi' ('if possible, want once more'). But that doesn't explain why people have adopted the phrase so widely.
One of my students suggests the image is of the yam leaf bowing down, to suggest humility. That strikes me as folk etymology, and it may not be the real origin of the phrase. But if people have that image, perhaps it might explain some of the popularity.
3 hours ago