"Have you eaten?"
(Note: This transcript has been edited for readability.)
"You talk to your mom on the phone, 'Did you have breakfast?' Necessarily you have to ask if they've had breakfast or lunch or dinner. But always ask, 'Did you eat? [laughing] Did you eat well?' So that's just a normal [way of] saying hi. So I guess it's from the poverty time, I think. And traditionally, historically, it is. It is all about being well-fed, I think. [laughing]"
In this video, a South Korean national describes why the phrase “Have you eaten?” is a common greeting.