In a crooked, bent, or dishonest manner; at an angle that is not straight.
From crooked plus the adverb-forming suffix -ly, the standard English pattern for creating adverbs from adjectives.
Interestingly, 'crookedly' can mean both 'in a physically bent manner' and 'in a dishonest way,' and this double meaning perfectly matches how 'crooked' itself shifted from purely physical description to metaphor for dishonesty.
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