A long time (especially a longer time than expected); ages(noun)
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Use "yonks" in a sentence
"I havent been on BP in yonks and now that my computer is back in working mode BP decides to not upload on my computer .. aaarggghhhh!!"
"Sultry voice belying her 20 years and lyrics – quirky doesn’t half describe them — makes her one of the best young musicians I have heard in yonks!"
"The only moan that I have about the book is that the Sceptre paperback version had print so small that it had me reaching for my reading glasses, and one of the least inspiring cover designs I've seen on a paperback in yonks."