Why does Kim Jong Min always make fun of Shin Jung Hwan with all in in "Love Letter"?

All in means all-in in gambling,

And there is a TV series in South Korea called ALL INgt; which is based on Cha Min-soo, a Korean professional gambler in the United States. Adapted from a novel.

That is, the word ALL IN is related to gambling

Shin Jung-hwan was caught in an underground casino at the beginning of this year and was even featured on the "9 O'clock News" ( Equivalent to our "News Network"), it was banned by the three major TV stations in South Korea and frozen by its company.

Then it was banned for a period of time, and it was not until the special episode of the heroine Saipan that she came back.

Later, Aunt Shen came back, so she always teased him about ALL IN

Aunt Shen would be embarrassed when ALL IN (gambling) was mentioned.