Adjective
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with the head foremost
"a headfirst plunge down the stairs"
"a headlong dive into the pool"
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excessively quick
"made a hasty exit"
"a headlong rush to sell"
Adverb
-
in a hasty and foolhardy manner
"he fell headlong in love with his cousin"
-
at breakneck speed
"burst headlong through the gate"
-
with the head foremost
"the runner slid headlong into third base"