What is the reference Kanbara made in episode 4 of Kyoukai no Kanata: Mini Theater special? At 01:53 of episode 4 of Kyoukai no Kanata: Mini Theater special, after Hiroomi said that Mitsuki was his only idol, this conversation happened (transcribed to the best of my ability):


  Kanbara: おい、博臣{ひろおみ}、そろそろやめとけよ。美月{みつき}がＭＫ[５]｛ご｝だぞ。
  
  Kuriyama: メガ盛｛も｝り 金平ごぼう、　ですか？
  
  Kanbara: そっ、それ、マジ？


If my guesses are right, Kuriyama mistook MK5 (emu-kei-go) as abbreviation for Megamori (large serving) Kinpira1 Gobou2, which is a Japanese dish.

1 A Japanese style of cooking. ja.Wikipedia. en.Wikipedia.
2 Burdock. ja.Wikipedia. en.Wikipedia.



Image from http://cookpad.com/recipe/1743175

My question is about "MK5" in Kanbara's line. While the line can be generally understood as Mitsuki was getting pissed off at Hiroomi, I don't get what Kanbara is making a reference of by saying "MK5". Any idea?
"MK5" means "maji kireru 5 byoumae" (マジで切れる５秒前), or being on the verge of losing it (really pissed off).