A "yonk" occurs as someone sneaks up behind another person, places one hand on the back of the yonkee's neck. Then grabs their forehead and proceeds to make a neck-breaking motion. This subsequently followed by saying (or yelling) some obnoxious sound effect. The intent is not to injure anything but pride.
1) The unit of 'Yonk' is known to be exactly 3 Months and 13 Days.
--> Used to describe a long period since you last saw someone or something. Usually termed 'Yonks' rather than a 'Yonk'.
BOB: "Gee Willy, I haven't seen you for Yonks!"
WILLY: "Yeah, must be about 3 Yonks actually."