headfirst

from WordNet (r) 3.0 (2006)

headfirst
    adv 1: with the head foremost; "the runner slid headlong into
           third base" [syn: {headlong}, {headfirst}]
    adj 1: with the head foremost; "a headfirst plunge down the
           stairs"; "a headlong dive into the pool" [syn:
           {headfirst}, {headlong}]

from The Collaborative International Dictionary of English v.0.48

Headfirst \Head`first"\ (h[e^]d"f[~e]rst`), Headforemost
\Head`fore"most`\ (h[e^]d`f[=o]r"m[=o]st`), adv.
   With the head foremost; -- of motion.
   [1913 Webster]

from Moby Thesaurus II by Grady Ward, 1.0

37 Moby Thesaurus words for "headfirst":
      a corps perdu, abruptly, carelessly, desperately, furiously,
      happen what may, hastily, head over heels, headforemost, headlong,
      heedlessly, heels over head, helter-skelter, holus-bolus,
      hotheadedly, hurriedly, hurry-scurry, impetuously, impulsively,
      like crazy, like mad, madly, overeagerly, overenthusiastically,
      overzealously, precipitantly, precipitately, precipitously,
      ramble-scramble, rashly, recklessly, slam-bang, slap-bang,
      slapdash, suddenly, wantonly, wildly