In a manner of chucking or throwing; with the motion or gesture of tossing something.
From 'chuck' (to throw) plus '-ing' (present participle form) plus '-ly' (adverbial suffix), creating an adverb for how something happens in a throwing motion.
This word is so rare that most English speakers have never encountered it, yet it's grammatically perfect—it shows that English lets you theoretically create almost infinite new adverbs, even if speakers never actually use them!
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