A girl that sometimes, often, likes to be called lesbian. These types of girls have the ability to manipulate other girls into doing what they want. Sometimes Yonks will want the manipulated girl to have them do sex stuff. Such as, get anal beads, dildos, BIG dildos, etc. Yonks are the most valuable types of girls. You'd definitely want a Yonk in your house!
A period of time equivalent to three weeks. Based on the fact that there is a term for one week, two weeks (a fortnight) and almost precisely four weeks (a month).
Because events are rarely important enough to know that they are approximately three weeks away, the term is more usually used informally as a way of exaggerating a length of time.
"The tournament starts in a yonk, so we'd best start practising."
In its more informal usage: "Ah, the project's not in for yonks, yet, so don't worry about it."