Hey,my question is posted below in red.
Sheldon: I need to know exactly what Leonard did to get you to
pop an emotional cap in his buttocks.(Q:What does it really mean? )Penny: What?
Sheldon: Again, urban slang, in which I'm beginning to get remarkable fluency.
Thank you for your time.
Wow! That's a first for me too. Let me take a stab at it. If I were to guess, since I'm not 100% sure, I would say that Sheldon is asking what Leonard did to Penny that caused her to "Blow up at him." In other words, Leonard said or did something and Penny "went off on him," literally exploded with/in anger. While "popping a cap in someone's buttucks (ass)" is definitely slang for shooting someone with a gun, I think the only common denominator between the two is the act of exploding. A bullet explodes from a gun when it is shot, and a person can explode when he/she looses his/her temper.