The word "speedingly" is spelled with a "-ingly" suffix, which indicates the adverb form of the root word "speedy". The pronunciation of "speedingly" in IPA phonetic transcription is /ˈspiːdɪŋli/, with emphasis on the first syllable. This word means "done or performed quickly", and is used in formal written English. While some may argue that the word is archaic or unnecessary, it remains a valid and proper part of the English language.
The word "speedingly" is derived from the root word "speed", which originates from the Old English word "spēd". In Old English, "spēd" meant "success" or "prosperity". Over time, the word evolved to also denote "swiftness" or "rapidity". The addition of the suffix "-ing" to "speed" forms the adverb "speedily", which means "quickly" or "swiftly". The further addition of the suffix "-ly" to "speedily" creates the adverb "speedingly", which is less commonly used today but carries the same meaning of "in a speedy manner" or "with great haste".