http://smbc-comics.com/comic/filth
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According to Green's Dictionary of Slang, the slang verb form of cock is attested in English as early as 1450 and in specifically Scottish sources by 1768. All I can figure is Macdonald was such a 'grandma' to use Tolkien's insult, that he either didn't know or couldn't imagine people taking it in a prurient way.
Bonus panel
