“New York’s water is so hard it comes at you with a switchblade”

“New York’s water is so hard it comes at you with a switchblade” is a joke that was probably told during New York City’s high crime days of the 1970s. Early citations of the joke haven’t been located, however.
 
 
NYC.gov
Water Hardness Information
The City provides information about water hardness through the New York City Drinking Water Supply and Quality Report.
 
Hardness is a measure of dissolved calcium and magnesium in water. Instructions included with some dishwasher models direct you to contact your local water department for information on local water hardness before operating the appliance.
     
Yahoo! Answers
Our water is so hard it carries a switchblade and doesn’t take any sh!t off anyone 😉
Sparky has a scar! · 7 years ago (Answer to a question that was asked on January 6, 2008.—ed.)
 
reddit
Distressed_Ocelot 2 points 3 years ago (Conversation started July 31, 2011.—ed.)
Where I live the water is so hard it carries a switchblade.
     
Vogue
Donna Karan / Fall 2015 RTW
FEBRUARY 16, 2015 6:20 PM
by MARK HOLGATE
(...)
Yet even when New York was, and is, at its toughest—and that can be pretty darn tough; remember that old joke about the place, even the water is so hard it comes at you with a switchblade?—Karan has also periodically sought out, and celebrated, the city’s softer, soulful side.