/ ˈhedlɒŋ; NAmE -lɔːŋ/ adverb
1.
with the head first and the rest of the body following
SYN head first :
She fell headlong into the icy pool.
2.
without thinking carefully before doing sth :
The government is taking care not to rush headlong into another controversy.
3.
quickly and without looking where you are going :
He ran headlong into a police car.
► head·long adjective [ only before noun ]:
a headlong dive / rush
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WORD ORIGIN
Middle English headling (from head + the adverbial suffix -ling ), altered in late Middle English by association with -long .