Yonks means a long time — a really long time, longer than you expected. 'I haven't seen him for yonks.' 'That shop closed yonks ago.' It is warm, casual, slightly funny, and it is British; Americans mostly don't say it, and many wouldn't know it. Australians and New Zealanders do use it. There are two things to notice about how it behaves. First, it always has the s, and it is not really counted — you never say 'three yonks'. Second, it lives in a small set of patterns: 'for yonks', 'in yonks', 'yonks ago'. You will very often find it with a negative: 'I haven't been there for yonks', 'We haven't spoken in yonks'. Saying 'I waited yonks' is possible but less common; British speakers would more often say 'ages'. In fact ages is the word to keep in your head: yonks and ages mean nearly the same thing and swap freely, but ages is far more common and safe everywhere. Yonks is the more colourful, more British cousin. It is informal, so keep it out of formal writing, but it is not rude at all — you can say it to a teacher or a boss in a friendly chat.

很久、好久（英式口語，相當於「好久沒…」的那個「好久」）