headfirst /ˈhɛdˈfɜːst/ (alsohead-first)

• 1.  

  ADV（副词） If you move headfirst in a particular direction, your head is the part of your body that is furthest forward as you are moving. 头向前地 [ADV after v]

  例：

  He had apparently fallen headfirst down the stairwell.

  他看上去是从楼梯井口头朝下摔下来的。