Definitions

1. adverb

in a headlong or impetuous manner

“He decided to enter the competition headfirst, without even knowing the rules.”

2. adverb

in a head-first manner, as if falling or diving head first

“The bungee jumper took the leap headfirst, feeling a rush of adrenaline.”

3. noun

a sudden or headlong plunge or dive

“The bungee jump was a headfirst plunge into the unknown.”