A young beaver; a small or juvenile beaver; something resembling a small beaver.
Beaver plus the suffix -kin (from Old English cyn and Germanic roots meaning 'child' or 'small'), used productively to create diminutives like lambkin or bumpkin.
The -kin suffix is one of the cutest in English because it turns any animal into a small version—lambkin, duckling, pupkin—and shows how people use language to express affection for small things, which is why it's still used in fantasy words like Tolkien's hobbits (though he didn't use -kin for them).
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