It is not spelled out in law. However I did find this:
Matter of Jamie D., 59 NY2d 589 (1983). Whether a "knife" is a "dangerous" knife may be determined on the basis of three alternative considerations: one, its own characteristics which show that it is primarily intended for use as a weapon; second, a modification, which converts what would otherwise be a utensil into a weapon; and third, the circumstances of the possession which may reveal that the possessor considers it a weapon and not a utilitarian tool.
- Gun Control in New York: New York's Firearms and Weapons Laws, Lee O. Thomas and Jeffrey Chamberlain
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