The South Korean language has numerous small details that don’t immediately translate into English, and one of them is the way people might address one another. Within BTS, each member will call any of his bandmates differently, depending on their age. However, there is one member, the youngest, Jungkook, who is almost always referred to by the nickname “Jungkookie.”
What happens, usually, is that younger men will call older men “hyung” — an informal honorific in the Korean language, that literally translates to “older brother,” which shows respect, but also affinity, to an older man you are close to. If you’re a younger woman referring to an older male friend, you would call him “oppa” even though it also translates to “older brother.”
Jungkook calls all his fellow BTS members “hyung” since he is the youngest of the seven, but no BTS member calls him the same. Although older men might call their younger friends “dongsaeng” (younger sibling) it’s not as commonly used as the opposite. Jin, SUGA, j-hope, RM, Jimin, and V often refer to Jungkook as “Jungkookie” — this is an affectionate way to call on their friend, and a good alternative to simply calling him by the very serious-sounding “Jungkook.” Sometimes, BTS will also call Jungkook “Kookie” or “JK.”
“Jungkookie” isn’t a nickname exclusive to the 25-year-old singer, though. Unlike “hyung” or “dongsaeng,” there is no age restriction to the use of the “ie” suffix. Just as long as two people are close to one another, it’s natural to call each other that type of diminutive. If you’re using it with someone older, you add “hyung” at the end. For example, Jungkook might refer to his bandmate RM, who is 3 years older than him, as “Namjoonie-hyung.”