Ahjinssi
Ahjinssi is a combination of the name Jin (A member of the kpop boy group BTS) and Ahjussi which means 'uncle,' but is the formal way to refer to a older man that you don't know well (kinda like 'Sir' in English). This started being used because when Jin is angry or flustered, he sounds like an old man and tends to yell or lecture the person bothering him about what they're doing.

Ahjinssi can be used for anyone who get's angry/flustered easily and makes a big deal out of whatever is bothering them(usually by yelling). It can also be used for someone who in general acts like an uncle/Dad, going strong with the dad jokes.
Ahjinssi: *Makes a dad joke*
SG: Can you stop being such an Ahjinssi
Ahjinssi: YAH I CAN DO WHAT I WANT!