The word “steam” can mean “to anger” and a person can have a “clammy” personality, as in the joke:
 
Q: How do you steam clams?
A: Make fun of their religion.
 
“Very few people realize it, but did you know you can steam clams by making fun of their religion?” was cited in a 1980 book. The joke probably originated in the newspaper comic strip B.C., where clams were frequently mentioned (and were a unit of currency). It is often stated that television host Johnny Carson (1925-2005) told the joke on The Tonight Show to make fun of Scientology. However, neither the B.C. cartoon nor the Carson joke has been located.
   
   
Wikipedia: B.C. (comic strip)
B.C. is a daily American comic strip created by cartoonist Johnny Hart. Set in prehistoric times, it features a group of cavemen and anthropomorphic animals from various geologic eras. B.C. made its newspaper debut on February 17, 1958, and was among the longest-running strips still written and drawn by its original creator when Hart died at his drawing board in Nineveh, New York on April 7, 2007.
   
Google Books
Bob Steele:
A Man and His Humor
Illustrated by Bob Steele
hartford, CT: Spoonwood Press
1980
Pg. 152:
Taboo
Very few people realize it, but did you know you can steam clams by making fun of their religion?
   
Google Groups: alt.brother-jed
The northeast
Steve Lamont
12/30/91
In article <1991Dec30….@uoft02.utoledo.edu>

.(JavaScript must be enabled to view this email address) (Dave) writes:
>I don’t get it… or I’m missing the metaphor or something…
>how do you boil, etc. a Christian ?
 
The same way you steam clams.  Make fun of their religion.
   
Google Groups: alt.religion.scientology
Johnny Carson on religion and steaming clams!
Patrick L. Humphrey
4/8/98
.(JavaScript must be enabled to view this email address) (Andreas Heldal-Lund) writes:
>I have no date but this is supposed to be a quote from
>Johnny Carson, stand-up monologue on NBC’s “The Tonight
>Show”:
>
>  “How do you steam clams? Make fun of their religion.”
 
Carson had a way with words…but if he _did_ say this as part of his monologue, it would have to have been prior to when he left the show in 1992. (Then again, maybe he knew something about Hubbard—Carson was a native of Norfolk, Nebraska…about 30 kilometers east of Tilden, and we all know who claimed to have been born _there_.)
 
—PLH, RumBunny patterning his act after Ed McMahon’s?  Hmmm…
 
The Commercial Appeal (Memphis, TN)
Letter: Docs, cut the stress
Oct. 22, 2013
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Bill Wilson, Cordova
One of the best “B.C.” comic strips years ago featured a question and answer:
 
“How do you steam clams?”
 
“Insult their religion.”
 
We can add “or their politics.”
   
Twitter
gamesphobia
‏@gamesphobia
“How do you steam clams? Make fun of their religion.” [Johnny Carson, stand-up monologue on NBC’s “The Tonight Show”]
6:27 AM - 3 Mar 2015