How it Works34 mins ago
knock bells
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For more on marking an answer as the "Best Answer", please visit our FAQ.It's usually given as 'seven' bells. Click http://www.wordwizard.com/clubhouse/founddiscuss.asp?Num=1749 for an answer. It is based on nautical language...bells being the method of timing aboard ship.
It may have been contrived through mispronunciation but in London I used to hear of "knocking seven bales" out of someone, this being short for knocking "seven bales of muck" out of them (the cleaner version). In less polite company you would hear "seven bales of s++t" and, more recently (though, thankfully, far less often), I've heard of knocking out "seven shades" of it.
Obviously these are likely to be loosely related to the versions given at Quizmonster's link - probably brought about through misquotation through time.