"headfirst" in the adjective sense
1. headfirst, headlong
with the head foremost
"a headfirst plunge down the stairs"
"a headlong dive into the pool"
"headfirst" in the adverb sense
1. headlong, headfirst
with the head foremost
"the runner slid headlong into third base"
Source: WordNet® (An amazing lexical database of English)
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