Source : Webster's Revised Unabridged Dictionary (1913)

Headfirst \Head`first"\, Headforemost \Head`fore"most`\, adv.
   With the head foremost.

Source : WordNet®

headfirst
     adj : with the head foremost; "a headfirst plunge down the
           stairs"; "a headlong dive into the pool" [syn: {headlong}]
     adv : with the head foremost; "the runner slid headlong into third
           base" [syn: {headlong}]