IPA: /ˈblʌstərɪŋli/
KK: /ˈblʌstərɪŋli/
In a way that is loud, aggressive, or full of strong emotions, often without real substance.
He spoke blusteringly, trying to impress everyone with his confidence.
Blusteringly is formed from "bluster" (from Middle English *blustren*, meaning to blow or roar) and the adverbial suffix "-ingly" (meaning in a manner of). The word describes an action done in a loud, aggressive, or boastful manner, akin to the sound of strong winds.
Think of someone who is 'blowing' or 'roaring' ('bluster') in a loud and boastful way, and the '-ingly' shows how they are doing it.