For years, I thought I'd heard others say, "I don't mean to knock on your idea, but..."—and it was definitely on, not down, although I've heard knock down as well—but now that I'm looking it up, I can't seem to find the idiom anywhere. The closest thing I've found is knock in the Urban Dictionary as meaning "to disregard, turn down; to ignore,"—but without the preposition on.

Have I misheard others all these years?—or have you heard the phrase used this way, too? If knock on isn't a real idiom, then is knock down the only alternative, or are there similar variations?

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    You "can't seem to find the idiom anywhere" because it's not an idiom. – Kris Oct 31 '13 at 13:10
  • @mplungjan: Can you add that as an answer so the asker can accept it? – MrHen Oct 31 '13 at 15:53
  • Ok, done - but it I would consider it GR and I think it will be closed as such – mplungjan Oct 31 '13 at 17:39

The usage is Knock, not knock on:

The free dictionary:

6. Slang To find fault with; criticize: Don't knock the food; it's free.



3: to find fault with always knocking those in authority

More here: http://www.onelook.com/?w=knock&ls=a

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    In U.S. English, "knock on" does appear idiomatically in connection with knock used in the sense of "a harsh and often petty criticism." Merriam-Webster's, in the link you provide, gives as an example "the knock on him him was that he couldn't handle the pressure." Since the knock is figurative, we might have expected the accompanying preposition to be "against" or "about" or "with regard to," instead of "on." – Sven Yargs Oct 31 '13 at 20:52
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    I do not agree. It could be written The knock, on him, was... – mplungjan Oct 31 '13 at 22:34

"Knock on" is indeed an acceptable way of saying "put down the idea/deride the idea/find criticism with the idea".

Though the term "knock" means to find those faults, one doesn't simply say they 'knock' an idea or 'knock down' an idea, which both mean different things. The idea is that they are deriding the idea by 'knocking on' it, or finding faults with it.

A similar expression would be "I don't mean to rain on your parade" to say you do not mean to ruin a person's day with bad news.

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    Yes one does say knock an idea. Don't knock it if you haven't tried it – mplungjan Oct 31 '13 at 22:36

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