In a manner that dangles or hangs loosely; with a dangling motion.
From 'dangling' (present participle of 'dangle') plus the adverbial suffix '-ly' (from Old English, converting adjectives to adverbs).
You rarely see this word in writing because English speakers prefer to say 'loosely' or just use 'dangle' as a verb — it's like the word that technically exists but hardly anyone actually needs.
Complete word intelligence in one call. Free tier — 50 lookups/day.