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Please see this example. The poster uses the word lemon in the last sentence. I understand in a general sense that this is being used to indicate something bad or more specifically of inferior quality.
Indeed, Roget's Thesaurus shows the following synonyms for lemon: failure, flop, junk, reject, and piece of junk.
It will be interesting to know how lemon came to mean an inferior quality.
As shown here, it might have something to do with "sucking the juice out of somebody" or "bad taste left in mouth."
This seems contradictory to me since lemon is fruit with great nutritional, medicinal and culinary value. Moreover, there are so many fruits out of which you suck the juice out. And lemon taste is never bad.