Martin Gardner in The Annotated Alice:
A species of arctic duck that winters in northern Scotland is called the calloo after its evening call, “Calloo! Calloo!”
More likely, as readers Albert L. Blackwell and Mrs Carlton S. Hyman each pointed out, Carroll had in mind two forms of a Greek word, “kalos,” meaning beautiful, good or fair. They would be pronounced as Carroll spells them, and would fit well the meaning of the line.
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