American Heritage Dictionary
(4)
Century Dictionary
(6)
GNU Webster's 1913
(2)
WordNet
(1)
Elsewhere on the web
A bigger factor is the even draw pressure applied to both sides of the arrow nock, as opposed to the upward pressure of a release under the nock.
Brackett's [King's Arms tavern], on the Neck, and endeavored to 'nock 'em up, to give us a dish of coffee; but failing, we backed about to the Punch Bowl [in Dock Square?], where, after long labors, we raised the house and got our desires gratified, and got home about four o'clock in the morning.— Boston 1775
They are not they must be on the Government population control nock off list.— Homeland Stupidity
Simply, a higher octane was developed for more "anti-nock" in cars with higher compression.

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