The literal meaning is "Five seconds from seriously losing it." It is generally a word used to describe someone who is 'on edge.' They are at the point just before having a total meltdown.
Person #1: Hey, buddy! I didn't want no goddamn mustard on ma' burger!
Person #2: You need to calm yer ass down.
Person #3: Hit the deck! He's MK5!
Person #1: (Explosion, resulting in the eatery getting leveled)
a car driven by people that want a slow rusty disappointing yet very reliable car, usually driven by balding middle aged men who claim the great diesel mileage is well worth the lack luster looks
god damn look how rusty that mk5 golf diesel is ian, its simply terrible
Mk56 is a common name for a Tsundere. Those nicknamed or named Mk56 are apart of the Mk series. They tend to differ between each model number however, 56 has survived the longest so far.