Noun. An intrinsically intertwined pairing built on immediate comfort and trust.
From the moment they met they were so closely connected and essential to one another that we knew for certain their relationship was joongdunk.
Verb. Chaotic and unhinged flirting between two people who already share a close relationship.
They were so enamored of one another, that they didn't realize their flirting had become joongdunking.