In Kim Dong-in's (김동인) short story "감자," what does 감자 mean?

 ANSWER: sweet potato

The Korean word for "potato" is 감자, but it comes from the Sino-Korean word 감저 (甘藷), which was used to refer to both "potatoes" and "sweet potatoes." In fact, since the Chinese character 甘 (감) means "sweet" and 藷 (저) can mean either "potato" or "sweet potato," the word 감저 (甘藷) literally means "sweet (甘) potato (藷)." The Sino-Korean words 남감저 (南甘藷 - "southern potato") and 당감저 (唐甘藷 - "Chinese potato") also refer to "sweet potatoes."

In Kim Dong-in's (김동인) short story "감자," written in 1925, there are the following passages:

칠성문 밖 빈민굴의 여인들은 가을이 되면 칠성문 밖에 있는 중국인의 채마 밭에 감자(고구마)며 배추를 도둑질하러, 밤에 바구니를 가지고 간다. 복녀도 감잣개나 잘 도둑질하여 왔다.

In autumn, the women in the slum outside Chilseong Gate would take baskets and go at night to the vegetable garden of a Chinese man, who also lived outside the gate, to steal potatoes (sweet potatoes) and Chinese cabbage. Bongnyo (복녀), too, would often go there to steal potatoes.

어떤 날 밤, 그는 고구마를 한 바구니 잘 도둑질하여가지고, 이젠 돌아오려고 일어설 때에, 그의 뒤에 시꺼먼 그림자가 서서 그를 꽉 붙들었다. 보니, 그것은 그 밭의 주인인 중국인 왕 서방이었었다. 복녀는 말도 못하고 멀찐멀찐 발 아래만 내려다보고 있었다.

One night, as she (Bongnyo) was standing to leave with a basket of sweet potatoes (고구마) she had just stolen, a dark shadow standing behind her grabbed her and held her tightly. She saw that it was Mr. Wang, the Chinese man who owned the field. Bongnyo was speechless and just stared vacantly down at her feet.

The above passages seem to confirm that the title of Kim Dong-in's short story "감자" should be translated into English as "Sweet Potato" or "Sweet Potatoes" since it was sweet potatoes (고구마) that were stolen from the Chinese man's garden. As can be seen HERE, there are others who seem to agree that the title "감자" should be translated as "Sweet Potato."

From "활용대옥편, Chinese Characters Dictionary"