In naver dictionary it says that panty or 팬티 are briefs, underpants, pants, panties, knickers, boxer short, boxers, or shorts.
Gu Ae Jung accidentally uses Dokko Jin’s panty to wipe the Medicinal Tonic she spills in Dokko Jin’s house. She goes there to replenish bottled waters in place of Jae Suk but ends up hiding from Dokko Jin when she sees him half naked, coming out of the shower. She finally gets caught trying to sneak out of his house holding his underpants.
Yoon Pil Joo calls her just as she’s about to leave Dokko Jin’s house. Hearing that she will meet Pil Joo later, a jealous Dokko Jin deliberately shouts to wash and leave his panty, delaying her from leaving.
“Wash my underwear before you go, so that my poor underwear doesn’t smell like the Medicinal Tonic!”
This scene is mirrored later on in the series when Dokko Jin enters Gu Ae Jung’s room and cleans up a medicinal tonic spill with her white shorts. (See: Face Cream and White Shorts)