In a manner resembling or characteristic of a cub; in a playful, ungainly, or youthfully immature way.
From cubbish (resembling a cub) + -ly (adverb-forming suffix). This follows the standard pattern of forming adverbs from adjectives by adding -ly, which comes from Old English.
Words like 'cubbishly' show how English builds adverbs—you can take almost any adjective and add -ly to make it describe how something happens. This system is so regular that we can even understand words we've never heard before!
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