“Raining like a (tall) cow pissing on a flat rock” is a popular expression in Texas and other southern states. The expression is similar to “raining cats and dogs,” meaning a lot of rain and splashing everywhere.
 
“He made more noise about it than a cow pissin’ on a flat rock” has been cited in print since at least 1949.
 
 
Google Books
Day Without End
By Van Van Praag
New York, NY: William Sloane
1949
Pg. 153:
Rodriguez was a good old boy and watched out for the men; but he made more noise about it than a cow pissin’ on a flat rock.
   
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Doc Dillahay
By Paul S. Powers
New York, NY: The Macmillan Company
1949
Pg. 44:
“It’s like cow when he lets go on a flat rock!” Joaquin yelled between the lightning strokes.
 
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From Here to Eternity
By James Jones
New York, NY: Scribner
1951
Pg. 165:
“It was rainin like a tall cow pissin on a flat rock, when I left there.”
     
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Road in the Wilderness
By Peter Pinney
Melbourne: Australasian Book Soc.
1952
Pg. 172:   
“Rain, rain, rain. Rains much more, be like a cow pissin’ on a flat rock.”
 
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New Country

By Allan R. Bosworth
New York, NY: Harper
1962
Pg. 248:
“Raining like a cow peeing on a flat rock!” Floyd rejoiced.
 
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Sometimes a Great Notion: A Novel
By Ken Kesey
New York, NY: Viking Press
1964
Pg. ?:
“It’s like if I don’t go now, then it might be shiver all night long waitin’ to get across; it’s rainin’, look out there at it come down like a cow wettin’ on a flat rock!”
 
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October 1988, Texas Monthly, pg. 198, col. 2:
... (“Everytime a cow pisses on a flat rock,” Ben said, “we get alarmed”) ...
   
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Texas Empire
By Matt Braun
New York, NY: St. Martin’s Paperbacks
1996
Pg. 271:
“Dark as pitch and raining like a cow pissin’ on a flat rock.”
 
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Raining like a tall cow.
January 27, 2010 12:28 AM
Where does expression “raining like a tall cow” come from?
 
Meteorologist Mike Pechner was quoted in the SF Chronicle saying “This is the time of year when it should be raining like a tall cow,” in reference to this being an El Nino year.
 
I am interested in what exactly this means and where such a strange expression comes from.
(...)
The common Southern saying is “raining like a tall cow pissing on a flat rock”.
Video aid (No longer available—ed.)
 
Now, imagine that same cow, only taller, and evacuating it’s bladder while standing over a large, flat rock commonly found in some farm terrains.
 
My grandfather raised dairy cows. I can vouch from personal experience that the pissing of a tall cow on a flat rock means damn-near endless amounts of piss coming out hard, and splashing the hell everywhere.
 
Put me off beef for a month, it did.
posted by magstheaxe at 5:02 AM on January 27, 2010