Over on Twitter, Neal Whitman notes that the adjectival phrase knock-on appears twice in the current issue of New Scientist (British) and wonders if it’s “a recent BrE innovation? Just BrE? Just recent? Neither?”
My answer: fairly recent and, until quite recently, almost exclusively British English. The “quite recently” means that for this blog’s purposes, it’s an on-the-radar NOOB. The OED calls the phrase “chiefly” British and defines it as “Being a secondary or indirect consequence of another action, occurrence, or event; knock-on effect n. a secondary, indirect, or cumulative effect.” First citation is from The Times in 1972: “They would be more than willing to move towards a minimum wage of about £20 a week..if they could be assured..that there would be no ‘knock-on effect’ in the differentials demanded by the rest of the labour force.”
As to derivation, although the term (apparently) has a meaning in rugby football, it’s more likely that the adjectival phrase comes out of physics, where it means “Ejected, produced, or caused as a result of the collision of an atomic or sub-atomic particle with an atom.” (“Knock-on protons produced by 3MeV neutrons would not..produce visible flashes.” Nature, 1971.)
Joining in the Twitter conversation, Lynne Murphy reports finding twenty-nine instances of knock-on in the Corpus of Contemporary American English, which boasts of containing 450 million words used by Americans in writing and on the air since 1990. My own search of COCA yielded twenty-seven hits, but that’s close enough for jazz.
At least half of the uses, I would judge, were written in American publications or uttered on American broadcasts by British people. However, fifteen of the twenty-seven have occurred since 2008, and Americans are responsible for a increasing number of these. For example, in 2011, Steve Coll wrote in a New Yorker blog, “The probable knock-on effect of a second Taliban revolution in Afghanistan would be to increase the likelihood of irregular Islamist attacks from Pakistan against Indian targets.” Georgetown professor Charles Kupchan wrote in Foreign Policy this past January, “[Citizens of industrialized countries] also expect their representatives to deal with surging immigration, global warming, and other knock-on effects of a globalized world.” And Annie Lowrey wrote in the New York Times in August: “Mr. Obama’s address coincided with the release of a White House report quantifying education job losses and detailing the knock-on effects, like bigger classrooms and shorter school years.”
The slowness with which the phrase has been adopted here relates, I would say, to the longstanding presence of perfectly adequate alternatives, side effects and unintended consequences, or, more simply, results. But never underestimate the appeal of a NOOB. I predict that knock-on effects will continue and indeed accelerate its ascent.
notoneoffbritishisms.com 
“Side effect” was indeed my first thought for an equivalent meaning. However, for precision, would in some cases (e.g., physics) a more appropriate connotation be “direct effect,” as in “cause and effect”? Or, is “knock-on” intended, unintended, or misused to cover /all/ resulting effects?
Talking of rugby (union) football here is the best try ever:
No knock-ons on this clip.
Agree about the try. In fact, there is a knock-on by Baba’s No 5 at 10’, albeit he is lying on the ground facing his own try line. The referee played advantage and the ball was gathered by the All Blacks.
Thanks for digging into this one! Curiosity, satisfied.
Well done, Peter for that clip.
Unmistakably the old Arms Park in Cardiff; hallowed ground in this part of the world where Rugby is taken very seriously.
Wales are due for another test match with the All Blacks tomorrow in the new Millennium Stadium, a stone’s throw (or drop kick) from the Arms Park where that wonderful full length run took place almost forty years ago..
A ‘knock on’ in Rugby is the illegal, (often unintended), forward passage of the ball in play, typically resulting from a fumble when passing the ball or loosing grip. It results in a scrum with the opposition given the ball to ‘put in’. It usually signifies a reversal of the flow of the game.
The ‘knock-on’ effect.
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To be pedantic, the match was played at the National Stadium often, but erroneously, referred to as the Cardiff Arms Park. The latter still exists and is the home of Cardiff RFC and, once again, the Cardiff Blues.
A ‘knock-on effect’ is not really a side-effect, it’s a direct result: as somebody says on Wiktionary, ‘”knock-on effect” is almost a direct translation of the late Latin repercussio (repercussion)’, and “repercussion” is a fair synonym. The images that seem to occur to Britons to explain the phrase are billiard/snooker/pool balls cannoning into each other, and lines of dominoes falling over, although, of course, that’s got its own name, as misused in the 1960s about South East Asia.
(BrE) I’d just like to agree with Martyn Cornell that a knock-on effect is not really the same as a side-effect. This is from the Oxford Advanced Learner’s Dictionary – “causing other events to happen one after another in a series” – and they give the example – “The increase in the price of oil had a knock-on effect on the cost of many other goods.”
In UK, I always hear ‘knock-on effect’ used in the sense of there being (logical) consequential effects to some action. However, the idiom doesn’t itself imply that those are good or bad (c.f. repercussions, which are generally assumed to be bad).
As Rugby Football pre-dates particles physics, it’s far more plausible the scientific usage came via a rugby-playing physicist.
Knock-on, knock-off.
I’ve recently become aware (in London) of increasing usage of “knock-off’, instead of “counterfeit”, but meaning the same.
Definitely an Americanism. The OED cites a 1966 quote in the NY Times: “Copying designs to sell for less has a name in the industry. It is called the ‘knockoff.’”
‘knock on effect’ is in a lyric by The Tragically Hip from the mid 90s, so it was definitely in Canada by then.