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According to Urban Dictionary (among other sources), the phrase insensitive clod - later a popular retort on Slashdot - originated in this Calvin and Hobbes comic published on February 14, 1986.
It's unclear how the phrase didn't make it into the poll for Best Meme in Slashdot's First 10 Years, but the ensuing thread has plenty of examples of its use.
Comic via this "10 Emotional Moments from Calvin & Hobbes" post on cbr.com.
