For those unfamiliar, when you pay for something in a Korean shop or restaurant, it's expected that you will give and receive the means of payment (and the invoice/bill/receipt) with one hand (generally the right hand) and hold the other hand visibly to your chest or upper abdomen.
I have asked multiple family members and friends about this tradition and gotten different answers, most of which seem apocryphal. Where and when did this tradition originate? Is it unique to Korea? Would love to know the answer for sure.
EDIT: Hey guys, appreciate the guesses and anecdotal responses here -- but that's kinda what I've already gotten from my friends. If you don't have a sourced answer with historical info, please skip it. Thanks!
This gesture is honorific, originating in Neo-Confucian ideals of social hierarchy which became prevalent during the beginnings of the Joseon dynasty.
Some of the endless habits are that the young must bow to the elderly when greeting them, they must always give and receive using two hands while the elderly can use just one, they must always wait for the oldest person to eat first until they can eat, and they must always talk to the elderly using honorific language. They cannot even drink alcohol in front of the elderly and must turn away to drink unless the elder says that it is acceptable to drink in front of them.
This practice transferred to retail employees, such as servers at restaurants, who use honorifics sometimes excessively.
While it's difficult to place a date on the first occurrence of this hand gesture, Korean language underwent a shift towards honorifics concurrent with the Neo-Confucian movement, so it's likely that it became widespread around that time.