1. What does it mean?
밀당 shortens 밀고 당기기, 'pushing and pulling.' In dating talk it describes alternating closeness and distance, often to keep another person interested.
Also written or said as 밀고 당기기, 밀당하다
2. The nuance
The term can be playful, admiring, or critical. A slow reply is not automatically deliberate 밀당; the person may simply be busy. Describing a pattern is different from recommending it, and direct communication can avoid the uncertainty it creates.
3. Examples
“답장을 일부러 늦추는 밀당은 나한테 안 맞아.”
Deliberately delaying replies to play hard to get isn't for me.
“둘이 마음을 숨기며 밀당하다가 결국 솔직하게 얘기했어.”
They kept hiding their feelings and going back and forth, then finally spoke honestly.
Sources
KoreanClass101: Korean language lessons
References support the meaning and usage. Example sentences are written for NoCapWiki.